RF signaling system and system for controlling the whereabouts of animals using same

ABSTRACT

An RF signaling system and system for controlling the whereabouts of animals utilizing same functions, in a preferred embodiment, by efficiently establishing well-defined electronic boundaries using small, battery powered transmitters transmitting constant envelope RF signals in a near-field regime by repetitively delivering trains of brief, low duty cycle pulses of electrical energy to a resonant circuit which includes an antenna at a low RF carrier frequency corresponding to the resonant frequency of the circuit. During intervals between pulses, the resonant circuit is effectively isolated from the circuit delivering the pulses to minimize loading effects. Further energy efficiency is achieved by interposing inactive interals between at least some successive transmissions of the RF signals represented by the pulse trains. The RF signals emitted by the transmitters are digitally encoded as are those emitted from a wire loop coupled to an optional wire loop unit to define a wire loop boundary. A unit affixed to the animal selectively administers appropriate aversive stimuli effective to deter encroachment of the boundaries in accordance with predetermined conditions based on the coding and timing of RF signals received from the transmitters and/or the wire loop. In the event of a break in the wire loop, the wire loop unit automatically switches excitation of the wire loop to an AM signal thereby facilitating location of the site of the break for repair purposes using a conventional AM radio receiver. 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

CROSS REFERENCE TO RELATED APPLICATIONS

Priority under 35 U.S.C. § 119(e) is hereby claimed to commonly-assignedU.S. Provisional Application No. 60/029,464, filed Oct. 25, 1996, andentitled "Low Power, Low Frequency Control of Animal Whereabouts" whichis expressly incorporated herein by reference in its entirety to formpart of the present disclosure.

Priority under 35 U.S.C. § 119(e) is also hereby claimed tocommonly-assigned U.S. Provisional Application No. 60/060,370, filedSep. 29, 1997 and entitled "Apparatus and Method for Transmitting anEncoded Signal Through a Metal Surface and Vending System IncorporatingSame" which is expressly incorporated herein by reference in itsentirety to form part of the present disclosure.

REFERENCE TO MICROFICHE APPENDIX

This disclosure includes a Microfiche Appendix containing computerprogram listings consisting in total of two (2) sheets of microficheincluding one hundred thirteen (113) total frames which are expresslyincorporated herein by reference in their entirety to form part of thepresent disclosure.

FIELD OF THE INVENTION

The invention relates to a novel RF signaling apparatus and method andto systems for controlling the whereabouts of pets or other animals byusing radio frequency transmissions to establish boundaries for animalsand by applying stimuli to the animals when they move into proximity ofsuch a boundary to deter them from traversing it. More particularly, thepresent invention relates to RF animal whereabouts control systems whichare highly efficient, consume little power, exhibit excellent immunityto noise and signal propagation anomalies, permit different animals inthe same vicinity to be subjected to different confinement conditionsand are readily adaptable for use with new or preexisting wire loopboundary installations.

BACKGROUND OF THE DISCLOSURE

Traditionally, the whereabouts of animals have been controlled byerecting physical barriers such as walls, fences or gates at the site ofa boundary the animal is to be prevented from crossing. Such barriersmust not only be high enough to prevent the animal from jumping over anddense enough to prevent the animal from passing through any gaps butalso substantial enough to withstand attempts by the animal to breachthe barrier by physical force. The latter requirement is a seriouslimitation in that in addition to requiring considerable time and laborto erect, substantial physical barriers are often impracticable due toshortage of materials suitable for their construction.

Those limitations have been overcome to some extent by the developmentof avoidance-inducing physical barriers, of which, barbed wire and highvoltage charged fences are well-known examples. Rather than relyingsolely on physical strength to defeat attempted breaches, animalsquickly learn that contact with such barriers is associated with anaversive stimulus such as being shocked or pricked by sharp barbs. Theytherefore avoid repeated or sustained attempts to breach them. Physicalbarriers of the avoidance-inducing type have permitted the fencing oflarge areas with the expenditure of only a fraction of the time, effortand materials which had previously been necessary. However,avoidance-inducing physical barriers also suffer from some importantlimitations.

First, like all physical barriers, posts and wires or other above-groundstructures are required to erect an avoidance-inducing physical barrier.In some applications, such as the confinement of household pets or guarddogs within a property line, these structures can be unsightly and aresometimes forbidden by deed restrictions or local regulations. Like allphysical barriers, avoidance-inducing physical barriers arenon-selective. A physical barrier sufficient to control the whereaboutsof a particular animal also tends to impede the ingress and egress ofpersons or other animals except at locations where a gate may beprovided. Moreover, the animal can traverse the barrier if the gate isinadvertently left open. Electrified or barbed wire fences intended forcontrolling the whereabouts of animals can also shock or injure persons,especially young children.

Various electronic systems which do not require the erection ofabove-ground barrier structures and which are at least somewhatselective in their operation are also known. In these systems,selectivity is achieved by equipping only the animal (or animals) whosewhereabouts are to be controlled with an electronic unit capable ofsensing when the animal moves into predetermined proximity of a definedboundary and then delivering one or more aversive stimuli to deter theanimal from traversing the boundary. Such stimuli commonly comprise anelectric shock either alone or in combination with an advance audibletone. A number of types of such electronic systems are known in theprior art. In most widespread use today are those of the "wire loop"type.

Various wire loop systems for controlling the whereabouts of animals areexemplified by U.S. Pat. Nos. 3,753,421 to Peck; 4,136,338 to Antenore;4,733,633 to Yarnall, Sr. et al.; 4,745,882 to Yarnall, Sr. et al.;4,766,847 to Venczel et al. and 4,967,695 to Giunta. In such systems,one or more continuous wire loops are routed along an arbitrary path todefine a boundary. In some cases the wires are run above ground, inothers they are buried. A controller connected directly to the loopgenerates an amplitude modulated (AM) signal which flows through theloop and causes an AM radio signal to radiate from the loop at apredetermined carrier frequency which is typically in the range of about8 KHz to about 20 KHz. These systems operate by simple on/off keying.When a battery-powered receiver unit affixed to the animal receives anAM signal at a magnitude indicating close physical proximity of theanimal to the boundary loop, the receiver initiates a generation of atone and/or shock to deter the animal from crossing the loop. When an AMsignal of such magnitude is not present at the receiver, no stimuli areapplied to the animal.

Many AM wire loop systems which operate in this manner have beeninstalled and are in operation at the present time. In certain of thesesystems, the animal's approach to the wire, as indicated by a receivedsignal strength above a predetermined threshold, initiates applicationof a first and relatively mild aversive stimulus (such as generation ofa tone) which terminates if the animal retreats from the boundary. If onthe other hand, the animal moves closer yet toward the loop, a higherthreshold of signal strength is exceeded and a stronger aversivestimulus such as an electric shock is administered in order to repel theanimal from the boundary as defined by the location of the wire.

The low RF frequency AM modulation used in prior art wire loop systemsprovides a number of advantages. Their low frequency AM radio signalspropagate through soil well enough to provide acceptable signal rangeabove ground. Low frequencies are also less likely to reflect fromnon-metallic obstructions which might otherwise create "shadows" or gapsin the boundary field. Another advantage of low frequency AM modulationis that if the wire loop is inadvertently cut or breaks, the localizedelectrical field emitted across the break enables one to easily locatethe break using an ordinary AM radio receiver. However, the lowfrequency AM wire loop boundary systems of the prior art also sufferfrom a number of disadvantages and limitations.

For example, AM systems are highly susceptible to electricalinterference from a variety of common sources including motor vehicles,motor operated appliances, light dimmers, and television sets. Suchinterference can cause an animal to be shocked even when it is not nearthe boundary. This undesired shocking is not only unnecessary andinhumane if persistent, but can also confuse the animal and interferewith its being trained to associated the aversive stimulus with theintended boundary. In order to provide an acceptable margin againstreception of false signals due to interference, AM systems of the priorart require a large received field intensity. Hence, the loop must bemade to radiate a commensurately intense field.

Because a wire loop does not act as an efficient AM antenna at the lowfrequencies at which AM wire loop systems operate, a relatively largecurrent, typically about two hundred (200) to about eight hundred (800)milliamps, must flow through the loop to generate a suitable boundaryfield. This not only consumes excessive power, making the system moreexpensive to operate but also precludes the possibility of powering thecontroller driving the loop using primary or backup batteries ofreasonable size and cost. As a result, the controller must be mounted ata location, usually indoors, near an A.C. power outlet and an A.C. poweroutage renders the entire system inoperable. When this occurs, theentire boundary is breached not merely a localized portion of it makingit much more likely that the animal will locate and exploit the breach.Also, since the closest point on the desired boundary may be somedistance away from an available A.C. outlet, installation of wire loopsystems is made more difficult, time consuming and expensive. Therelatively large current flowing through the loop also creates acommensurately large low frequency electrical field. Some have suggestedthat such fields may pose health risks to humans or animals.

Installing the wire loop represents a substantial portion of the cost ofa wire loop system. As noted above, a continuous length of wire must berun from the controller, around the desired boundary and back to thecontroller which is usually located at an indoor location remote fromany point on the desired boundary. The wire must be installed around orthrough any intervening walls or other obstacles. Even with specialequipment built for the purpose, it is not a trivial task to bury a wireloop encompassing the perimeter of a large property. Installation isfurther complicated by the necessity of twisting the loop wires togetherwherever they must pass through locations where no boundary field isdesired. Since the currents in any twisted portions of the wires flow inopposing directions, their fields cancel sufficiently that the unitaffixed to the animal does not initiate application of an aversivestimulus even when the animal is nearby. Thus, by twisting portions ofthe loop together, a boundary located remotely from the controllerand/or one having two or more distinct portions lying physicallyseparated from one another can be formed using a single loop of wireconnected to a single controller.

Remote broadcast systems are another class of electronic system known inthe prior art for controlling the whereabouts of animals. These do notrequire installing a loop of wire to define a boundary. Instead, aboundary is established by broadcasting an RF signal from a centrallocation toward an intended outer perimeter boundary. The location ofthe boundary is defined based on the strength of that broadcast signalas sensed by a unit affixed to the animal. For example, U.S. Pat. No.5,067,441 to Weinstein describes an animal restraining system includinga radio frequency transmitter, a transmitting antenna located inside anarea in which the animal is to be restrained and a collar unit worn bythe animal. A coaxial cable is run between the transmitter unit and thetransmitting antenna. When the animal strays from the transmittingantenna a distance sufficient to permit the signal strength received bythe collar unit to fall below a predetermined level, a first aversive ofstimulus, such as a beeping tone, is generated. If the animal straysfurther from the antenna by a distance sufficient to cause the signalstrength to fall below a second predetermined threshold, a strongerstimulus such as a shock is administered to the animal to deter itsdeparture from the area. A similar system is described in U.S. Pat. No.4,898,120 to Brose.

A fundamental shortcoming of remote broadcast type systems forcontrolling animal whereabouts is that the collar unit worn by theanimal does not detect proximity of the animal to a structure or objectwhose physical location reliably indicates the location of the intendedboundary. Instead, such systems rely on measuring signal strength as anindicator of the distance the animal from a transmitting antenna whichmay be located a considerable distance from the boundary. Consequently,that indication is not always reliable. One reason is that because suchsystems operate in a far-field regime, the magnitude of the fielddecreases only in proportion to the square of distance from thetransmitting antenna and does not vary significantly over distances onthe order of several feet. Another reason is that the strength of thereceived signal can change due to constructive and destructiveinterference generated by signal reflections, shadowing by metallicobjects and other uncontrollable variations in local receptionconditions. Since local reception conditions can fluctuate, the size,shape and location of the boundary loci at which stimuli will beadministered can also fluctuate. For example, if the signal path betweenthe transmitting antenna is temporarily altered by an automobile whichpulls into one's driveway, the animal may receive a shock even if theanimal remains within an intended perimeter boundary. Remote broadcastsystems also tend to require significant amounts of electrical power andthus, like wire loop systems, do not lend themselves to batteryoperation. This disadvantage stems in part from the need to broadcast asufficiently strong signal to the most remote portion of the boundaryand renders these systems, like AM loop systems, vulnerable to A.C.power failures.

Remote broadcast systems are also limited with respect to the sizes andshapes of perimeter boundaries they can define. While generally circularboundaries or ones conforming to the radiation pattern of a particularantenna can be implemented, continuous perimeter boundaries encompassingjutting regions or other well defined irregularities would be extremelydifficult, if not impossible to establish using a remote broadcast typesystem. Another limitation of such systems is that because signalstrength values are not unique to individual locations within the fieldof the transmitter, they are not well suited for excluding an animalonly from arbitrarily located discrete positions, such as the site ofone's prized rose bush for example. While wire loop systems offergreater flexibility and predictability of boundary shape, they aresubject to the problems and limitations described above.

Another significant limitation of both the wire loop systems and theremote broadcast systems described above is that they are only capableof defining boundaries whose positions remained essentially fixed. Priorart U.S. Pat. No. 5,241,923 to Janning described for the first time ananimal whereabouts control system which, while suitable for definingdiscrete and/or continuous fixed boundaries, was also capable ofdefining boundaries which moved with a mobile agent such as a child oranother animal so that a particular animal such as a dog could be keptseparated from child or other animal while otherwise allowing both dogand child complete freedom of movement. The systems described in Janning'923 operated at relatively high frequencies such as 915 Mhz where highantenna efficiencies could be obtained so as to reduce powerrequirements sufficiently to permit the system to operate on batterypower. In such a system, individual active or passive high frequencytransponders could be placed singly to exclude the animal from aspecific location or arranged in mutually spaced arrays to form closedor partially closed continuous perimeter boundaries of virtually anydesired size and shape. The transponders could be encapsulated forburial in the earth outdoors or packaged for placement beneath carpets,furniture cushions or area rugs or near entrances to rooms from which ananimal was to be excluded.

Being of small size, light weight, and completely self-contained, thetransponders disclosed in Janning '923 required no external wiring.Those transponders could also be provided with adhesive backing or withclips, pins or other attachment devices for securing them at a desiredfixed location or to a mobile agent such as a child, an automobile oranother animal which one might desire keeping separated from aparticular animal. Using a collar or other suitable means of attachment,there could be affixed to the latter-mentioned animal a battery poweredunit incorporating a receiver coupled to a stimulator for delivering atone and/or a shock to the animal being controlled. Mounted eitherremotely or within the battery powered unit affixed to the controlledanimal was a transmitter which generated an incident signal in the formof intermittent bursts of continuous wave (CW) energy at 915 MHz. Uponreceiving this incident signal, the transponders would generate a returnsignal. When the distance separating the animal from one of thetransponders became sufficiently close, the return signal would bepicked up by the receiver affixed to the animal and cause the stimulatorto deliver an appropriate aversive stimulus. Optionally, one or more ofthe transponders could be provided with a delay line or surfaceacoustical wave device which could be used to encode the return signalso as to identify to the receiver the particular transponder from whichthe return signal is emanated so as to enable different animals to besubjected to different boundary constraints by a single system.

While representing a major breakthrough in the art due to theirunprecedented flexibility, effective performance and simple, low-costinstallation, systems for controlling animal whereabouts usingtransponders as disclosed in Janning '923 were subject to limitations.In particular, placement of transponders at certain locations, such asthose adjacent or wholly or partially surrounded by metallic surfaces,sometimes resulted in blocked reflected or otherwise anomalous signalpropagation manifested in undesired variations in distance sensitivity(i.e., boundary range) or the complete inability to establish aneffective boundary at such locations.

Like all other known prior art systems (except for wire loop systemsthemselves), the systems described in Janning '923 also lacked theability to function with a wire loop boundary. Accordingly, they couldnot be used to replace or upgrade existing AM wire loop systems ofwhich, despite the disadvantages noted above, large numbers have beenand continue to be installed in the field.

SUMMARY OF THE INVENTION

In view of the foregoing problems and limitations of the prior art, itis an object of the present invention to provide an RF signaling systemcapable of generating and broadcasting an RF signal with high energyefficiency.

It is another object of the invention to provide an RF signaling systemwhich provides a high degree of immunity from noise and reception offalse signals.

It is yet another object of the invention to provide a system forcontrolling the whereabouts of animals utilizing the RF signaling systemof the invention to provide highly efficient operation affording longbattery life.

A further object of the invention is to provide a system for controllingthe whereabouts of animals in which the RF signaling system of theinvention is adapted to provide extremely reliable data communications,even in the vicinity of metallic surfaces, with high immunity from noiseand reception of false signals.

Another object of the invention is to provide a system for controllingthe whereabouts of animals utilizing the efficient RF signaling systemof the invention operating in a near-field regime to establish stableand distinct electronic boundaries which are unobtrusive, do not requirethe erection of physical barriers or other extensive installation, whichmay either be discrete or continuous in form and which may either befixed or movable in location.

A further object of the invention is to provide a system for controllingthe whereabouts of animals which fulfills the foregoing objectives whilebeing additionally capable of being used to establish a wire loopboundary in either new or preexisting wire loop installations and thusbe capable of retrofitting, upgrading and/or expanding a wire loopboundary system easily and at low cost.

A further object of the invention is to provide a system for controllingthe whereabouts of animals which is highly energy efficient permittingthe entire system, including any wire loop boundary portions thereof, tooperate for sustained periods entirely from battery power and thusremain fully effective in the event of an A.C. power outage.

A further object of the invention is to provide a system for controllingthe whereabouts of animals which fulfills the foregoing objectives whilefacilitating location of the site of any break in a wire loop boundaryusing a conventional AM radio.

The invention provides an RF signaling system which operates withextremely high energy efficiency. In accordance with one importantaspect of the invention, this is achieved by providing a transmitterwith a resonant circuit, such as a tank circuit, which includes anantenna and which is tuned to resonate at a desired RF carrier frequencyand by delivering to the antenna circuit a train of brief pulses ofelectrical energy repeated substantially at the aforementioned RFfrequency. Upon delivery of each pulse, the antenna is caused toresonantly ring, thereby radiating energy in the form of an RF signal atthe desired RF carrier frequency. The duration of each driving pulse ispreferably as brief as possible to minimize damping of the antenna andthus, facilitate maximum radiation of RF energy. The duty cycle of thepulse train is ideally as small as possible while still providing pulsescontaining sufficient energy to deliver at the RMS RF power levelrequired by a particular application. This duty cycle is preferably lessthan about five percent (5%) and is most preferably less than about onepercent (1%). Where higher levels of transmitted power are required, theduty cycle of the pulse train may be increased at the expenses of someefficiency but should not exceed about fifteen percent (15%).

The invention is to be contrasted with prior art RF drive circuits wouldtypically drive the output tank or antenna with an approximately 50%duty cycle (square wave). In such a case the output transistors of thedrive circuit would be in conduction 50% of the time. If sinusoidalexcitation were used, the output transistors could be in conduction upto 100% of the time. During such time, the output transistors would beloading the output tank with their output impedance, which could bequite low. In contrast, the present invention drives the output tankwith very short pulses of current. The remainder of the time, thecircuit is free to "ring" at its resonant frequency. This operation isanalogous to striking a tuning fork or church bell with a clapper tomake it "ring". If the clapper were held against the bell for a largepercentage of the time, most of the acoustical energy would bedissipated by the clapper. Instead, a single impulse hit by the clapperis the most efficient way to generate the most sound with the leastamount of input energy. Likewise in the present invention the "clapper"(current pulse) is of very short duration in order to increase theefficiency of radiated energy and greatly reduce the amount of energydissipated by the drive transistors.

According to another important aspect of the invention, loading of theresonant circuit by the drive circuit which would otherwise damp theresonant ringing of the antenna and thereby decrease the energyefficiency of the system is substantially avoided by isolating theantenna circuit from the drive circuit during at least a portion, andpreferably substantially the entirety, of the intervals between thedriving pulses. This is achieved by providing an isolation circuit whichis operably interposed between the drive circuit and the resonantcircuit.

In the preferred embodiment, the isolation circuit is implemented usinga pair of field effect transistors (FETs) connected mutually in parallelwith one another between the drive circuit and the resonant circuitwhich includes the transmitting antenna. These FETs are selected toexhibit extremely low impedance when in conduction and thus facilitatedelivery of high-current drive pulses to the resonant circuit withlittle forward loss. During the intervals between drive pulses however,both FETs are in a cutoff state in which they exhibit a nearly infiniteoutput impedance. This serves to effectively decouple the resonatingantenna from the drive circuit, thus electrically isolating the resonantcircuit from any loading influence of the drive circuit.

The efficiency achieved by use of these aspects of the invention can beillustrated by considering for example an RF signal to be transmitted atten kilohertz (10 KHz). A tank circuit including the antenna of thetransmitter is tuned to resonate at 10 KHz. One microsecond high energypulses are fed to the antenna at the desired RF frequency (i.e., a 10KHz rate) from a drive circuit by way of an isolation circuit whichexhibits a high output impedance during the intervals between pulses.Under such conditions, it can be appreciated that the circuitry drivingthe antenna draws current only 1% of the time (a 1% duty cycle). Incontrast, a typical prior art system such as an AM system broadcastingat the same frequency with 100 Hz square wave modulation would drawcurrent 50% of the time. The invention thus provides a comparative 50:1reduction in drain on the battery or other power source supplying thetransmitter thereby significantly increasing the ratio of radiated RFpower to D.C. power consumed by the system. Because the drive circuit iseffectively coupled to the antenna only during the brief intervalsduring which the drive pulses occur and is effectively decoupledtherefrom during the overwhelmingly major portion of the cycle, thedrive circuit loads the resonant circuit only for a correspondinglybrief portion of the cycle. Thus, the invention not only draws lesspower but also permits the transmitting antenna to ring without beingsubjected to significant damping due to loading effects of the drivecircuit in order to maximize the efficiency of the conversion of D.C.power to radiated RF power.

Other important aspects of the invention relate to systems forcontrolling the whereabouts of animals. In a preferred embodiment, theefficient RF signaling system of the invention is employed in a systemusing one or a plurality of small, battery-powered transmitters totransmit an RF signal establishing a boundary in the vicinity of thetransmitter. A unit affixed to the animal includes a receiver capable ofdetecting the RF signal. Upon determining, based on information derivedfrom the RF signal, that at least one predetermined condition indicatingencroachment of the boundary has been satisfied, the unit delivers atleast one aversive stimulus to the animal to deter such encroachment.These transmitters preferably utilize the RF signaling system of theinvention to provide extremely long operating life using a small, lowcost battery.

As applied to such a system, the invention achieves even furtherimprovements in energy efficiency by recognizing that an electronicboundary need not be present continuously without interruption in orderto remain fully effective. According to this aspect of the invention,the RF signal establishing a boundary for the animal is interrupted byinactive intervals during which very little energy is consumed by thetransmitter. In the preferred embodiment, the inactive intervals are inthe range of about thirty to about one hundred milliseconds. However,for certain applications, particularly those involving indoorboundaries, much longer intervals, up to about five (5) seconds orlonger can be used.

According to another important aspect of the invention, the transmittersoperate in a near-field regime transmitting an RF signal whose magneticfield component is not only much larger than its electrical fieldcomponent, but which also exhibits a magnetic field whose magnitudedecreases substantially in proportion to the cube of increasing distancefrom the transmitter. To do so, the transmitters operate at a lowfrequency, preferably about ten kilohertz (10 KHz) or less and mostpreferably about eight point one nine kilohertz (8.19 KHz), and includea transmitting antenna which is electrically small (i.e. of a size tofit inside a radiansphere). Because the magnitude of the detectableboundary field established by such an RF signal decreases so rapidly asdistance from the transmitter increases, the boundary field is not onlywell localized, but is also sharply defined and therefore, positionallydistinct. Moreover, because the field is predominantly magnetic incharacter and is established at such a low frequency, it is highlyimmune to blockage, reflections and distortion due to the presence ofnearby metallic objects. These characteristics afford establishment ofan electronic boundary capable of being detected positively, reliablyand repeatably at a substantially unique locus of points in the vicinityof the transmitter. To even further enhance reliable detection of the RFsignal, such as one defining an electronic boundary, a further aspect ofthe invention contemplates use of a modulation scheme in which theamplitude of the RF signal is bounded by an imaginary envelope ofsubstantially constant magnitude at all times when the RF signal isactively transmitted.

Yet another object of the invention is to provide a system forcontrolling the whereabouts of animals in which a unit affixable to ananimal is capable of administering an aversive stimulus in response toencroachment of a boundary established by RF signals emanating eitherfrom a wire loop or from one or more small, battery powered transmittersof the type referred to above. This is achieved by providing, inaddition to such transmitters, a wire loop unit connectable to a wireloop whose path traverses the midst of a desired wire loop boundary. Thewire loop unit excites the wire loop with radio frequency electricalenergy and causes the wire loop to emit a constant envelope RF signalwhich the unit affixed to the animal can detect and respond to in thesame manner as an RF signal emitted by any of the transmitters.

Since the RF signal which the wire loop unit generates and applies to awire loop to establish a wire loop boundary is not an amplitudemodulated (AM) signal, it cannot be used to locate the site of a breakin the wire loop. Accordingly, the wire loop unit includes circuitrywhich continuously monitors the continuity of the wire loop. In theevent a break in the wire loop is detected, an audible and visibleindication of the break is provided. At the same time, the wire loopunit decouples from the wire loop the RF signal which establishes theboundary and couples the wire loop to a source of amplitude modulated(AM) radio frequency electrical energy which enables one to locate thesite of the break for repair purposes using a conventional AM radioreceiver.

Another object of the invention is to provide a system for controllingthe whereabouts of animals in which the RF signals referred to above areencoded such that the unit affixed to the animal can discriminate theboundary established by a wire loop from one established by any of thetransmitters and/or can discriminate between the boundaries establishedby different ones of the transmitters. This enables different animals tobe subjected to different boundary constraints and permits the form,intensity and/or other parameters associated with the stimulationdelivered by the unit can be selected in accordance with the location ofthe particular boundary encroached upon by the animal.

According to the preferred embodiment, this is achieved by encoding theRF signals emanating from the wire loop as well as those emanating fromeach of the transmitters with an assignable digital code. The unitaffixed to each animal to be controlled is programmed to determinewhether or not to apply aversive stimulation when an RF signal isreceived from either the wire loop or one of the transmitters based onthe particular digital code associated with that RF signal. Byprogramming different ones of such units to respond to different Booleancombinations of these digital codes, different animals, even ones in thesame general physical area, can be simultaneously subjected to differentboundary constraints. The nature of the stimulation applied in responseto reception of a particular digital code by a particular unit affixedto an animal can also be preprogrammed.

These and other objects and advantages of the invention will becomeapparent to the person of ordinary skill in the art upon review of thefollowing detailed description of a preferred embodiment taken inconjunction with the appended drawings in which like reference numeralsdesignate like items.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a preferred embodiment of asystem for controlling the whereabouts of animals in accordance with thepresent invention in a typical installation.

FIG. 2A is a circuit schematic illustrating the electronic circuit ofeach of the transmitters shown in FIG. 1.

FIG. 2B is a continuation of the circuit schematic of FIG. 2A.

FIG. 3 is a timing diagram illustrating the operation of the transmittercircuit of FIG. 2A and FIG. 2B.

FIG. 4 is a timing diagram of expanded time scale relative to FIG. 3further illustrating the operation of the transmitter circuit of FIG. 2Aand FIG. 2B.

FIG. 5 is a top plan view of the antenna of the transmitter circuit ofFIG. 2A and FIG. 2B.

FIG. 6 is a cross-sectional view through the core of the antenna of FIG.5 as viewed from line 6--6 of FIG. 5.

FIG. 7 is a circuit schematic illustrating the receiver of thereceiver/stimulator units of FIG. 1.

FIG. 8 is a circuit schematic illustrating the quadrature demodulator ofthe receiver/stimulator units of FIG. 1.

FIG. 9 is a circuit schematic illustrating the control and stimulationsubsystem of the receiver/stimulator units of FIG. 1.

FIG. 10 is a timing diagram illustrating the administration of aversivestimuli by the control and stimulation subsystem of FIG. 9.

FIG. 11A is a circuit schematic illustrating the wire loop unit of FIG.1.

FIG. 11B is a continuation of the circuit schematic of FIG. 11A.

FIG. 11C is a diagram illustrating the aligning relationship of FIG. 11Aand FIG. 11B.

FIG. 12 is a flowchart illustrating the programming of themicroprocessor of the wire loop unit of FIG. 11A and FIG. 11B.

FIG. 13 is a flowchart illustrating Main Routine executed by themicroprocessor of the control and stimulation subsystem of FIG. 9.

FIG. 14 is a flowchart illustrating the Motion Check subroutine of FIG.13.

FIG. 15 is a flowchart illustrating the Detect subroutine of FIG. 13.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated an installed system 10 forcontrolling the whereabouts of an animal 12 according to the principlesof the present invention in the vicinity of a dwelling 13 andsurrounding areas. System 10 includes at least one receiver/stimulatorunit 14 which may be mounted on a collar 15 or other suitable means ofremovably affixing unit 14 to animal 12. System 10 further includes oneor more small, lightweight battery powered transmitters 17. Eachtransmitter 17 intermittently emits a digitally encoded, low frequencyRF signal 18 capable of being received by receiver/stimulator unit 14 inorder to establish an electronic boundary 20 which generally surroundseach transmitter 17. Transmitters 17 are preferably packaged inside asealed case of plastic or other suitable material and/or are potted topermit them to be used outdoors, either above ground or buried, as wellas indoors. Transmitters 17 can be packaged and positioned in the samemanner as taught with respect to the transponders described in commonlyowned prior art U.S. Pat. No. 5,241,923 to Janning, which is expresslyincorporated herein by reference in its entirety to form part of thepresent disclosure.

According to the invention, system 10 also includes an optional wireloop unit 21 capable of being connected to a newly installed orpreexisting wire loop 22. Wire loop 22 acts as an antenna which emits adigitally encoded, low frequency, RF signal 18 generated by wire loopunit 21 for the purpose of establishing an electronic wire loop boundary23 which generally surrounds wire loop 22. While wire loop 22 istypically buried a few inches beneath the soil in order to provide forits concealment and mechanical protection, all or portions of it may beinstalled above ground, either indoors or outdoors, to establish one ormore wire loop boundaries 23 of arbitrary size and shape in a desiredlocation. Wire loop 22 may be twisted in the manner illustrated at area26 in order to cancel the emitted field and thereby provide for desireddiscontinuities in wire loop boundary 23.

Animal 12 may roam freely unless and until it moves into sufficientlyclose proximity of either wire loop 22 or any of transmitters 17 as toenable receiver/stimulator unit 14 to detect the digital code associatedwith RF signal 18. When this occurs, unit 14 determines whether one ormore predetermined conditions associated with that code is satisfied. Ifno such condition is satisfied, unit 14 does not provide any form ofstimulation and animal 12 remains free to roam about without beinginfluenced by system 10. If such a predetermined condition is satisfiedhowever, unit 14 responds by administering one or more aversive stimulito animal 12 to deter it from traversing the boundary 23 or 20 which hasbeen approached.

System 10 affords a high degree of flexibility in establishing effectiveboundaries of virtually any size, shape or location. Although capable ofdefining boundaries indoors and/or in relatively small areas, the use ofa wire loop unit 21 in combination with a wire loop 22 is particularlywell suited to the establishment of large outdoor boundaries such as oneparalleling the perimeter of a property. By providing twisted areas suchas the one illustrated in FIG. 1 at area 26, a wire loop boundary 23located a considerable distance from wire loop unit 21 and/or onedefining two or more distinct boundary areas physically separated fromone another can readily be provided using a single wire loop unit 21connected to a single wire loop 22. Among other important distinctionsover the prior art, the invention is novel in that the samereceiver/stimulator unit 14 used to control the whereabouts of an animal12 using a wire loop 22 can also be made responsive to the electronicboundaries 20 established by one or more discrete transmitters 17. Theability to establish boundaries using discrete transmitters 17 alone orin combination with a new or preexisting wire loop 22 affords system 10unprecedented flexibility permitting system 10 to replace, upgrade orexpand prior art AM wire loop boundary systems at low cost and withminimal installation.

As FIG. 1 illustrates, transmitters 17 can be positioned at desiredlocations within a dwelling 13 or other structure so as to excludeanimal 12 from specific locations therein, such as depicted at areas Aand B which correspond, respectively, to the foot of a stairway 29 andthe entrance of a room 30. Other areas or items such as a rug or a pieceof furniture may be similarly protected if desired. In addition or inthe alternative, one or more transmitters 17 can be positioned toestablish boundaries 20 at discrete fixed outdoor locations, such as thelocations C and D indicated in FIG. 1. Such outdoor locations maycorrespond for example to a garden or outdoor dining area. Transmitters17 can also be affixed to a mobile agent such as an automobile, anotheranimal or a child 31 which animal 12 is to be deterred from approaching.By placing a plurality of transmitters 17 in an open or closed array,such that at least a portion of the boundaries 17 associated withadjacent transmitters 17 at least partially overlap one another,continuous open or closed linear boundaries of virtually any desiredshape and size, such as the linear boundary 33 illustrated in FIG. 1 canalso readily be formed for purposes of confining animal 12 within anarea E such as a portion of a backyard. Animal 12 may thus roam freelywithin backyard area E inside of boundary 33 except for any locationswhere its passage may be forbidden by any boundaries 20 such as thoseestablished by the transmitters 17 present at locations C and D of FIG.1., or those attached to a child 31.

System 10 affords further flexibility in that receiver/stimulator unit14 can be configured in the field to select a type and intensity ofstimulation most appropriate for a given animal 12. The preferredembodiment for example can be configured to administer either an audibletone only or an audible tone in combination with either a less intenseor more intense, yet harmless, electrical shock. For example, thereceiver/stimulator unit 14 worn by animal 12, which might be a large,aggressive animal, can be configured to administer a tone and a moreintense electrical shock. System 10 can also include more than onereceiver/stimulator unit 14 in order to control the whereabouts of aplurality of animals simultaneously. As FIG. 1 illustrates, a secondanimal 12' can be equipped with a second receiver/stimulator unit 14'configured to administer only an audible tone or whichever of the otheravailable types of aversive stimulation may be most appropriateconsidering the size and temperament of animal 12'.

System 10 also affords flexibility in that it permits different animalsto be subjected to correspondingly different boundary constraints.Different boundary constraints can be imposed not only to confinedifferent animals to or exclude different animals from mutuallyseparated areas but also to control the whereabouts of animals inphysical areas which partially or entirely overlap. This is achieved byassigning each transmitter 17, as well as optional wire loop unit 21,with an identifiable code. This code is transmitted by way of RF signal18 and used to determine the administration of aversive stimulationaccording to Boolean criteria applied by each particularreceiver/stimulator unit 14 or 14'.

By way of example, the system 10 of FIG. 1 can be configured to confineanimal 12 to area E, excluding it entirely from the interior of dwelling13 as well as from child 31 and outdoor locations C and D. At the sametime, second animal 12' may be permitted contact with child 31 andallowed to roam freely anywhere within wire loop boundary 23 with theexceptions of room 30 and the second floor of dwelling 13. This can beachieved in several different ways including by identifying the RFsignals 18 emanating from wire loop unit 21 as well as those emitted bythe transmitters 17 located at the foot of stairway 28 and the entranceof room 30 with an identical first code. The receiver/stimulator unit14' affixed to second animal 12' is programmed to respond to that firstcode upon detection of same by administering an appropriate aversivestimulus to animal 12' without responding to an RF signal 18representing any other code. At the same time, all the othertransmitters 17 of system 10, including those on boundary 33, those atlocations C and D and the one affixed to child E, emit an RF signal 18encoded with a second code which is different from the first code. Theabove-described boundary constraints could then be achieved simply byprogramming the receiver/stimulator unit 14 worn by animal 12 tostimulate animal 12 only in the event the second code is detected.Preferably, however, unit 14 would also be programmed to likewiserespond to detection of the first code. This would ensure that animal 12would at least be confined within perimeter boundary 23 and would beexcluded from room 30 as well as the second floor of dwelling 13 ifanimal 12 somehow escaped backyard area E. Under normal circumstanceshowever, the transmitter 17 located at the northeast entrance ofdwelling 13 and those forming linear boundary 33 would operate toexclude animal 12 from dwelling 13 entirely.

An important aspect of the present invention relates to a novel andimproved radio frequency (RF) signaling apparatus and method which areincorporated in system 10 to establish highly energy efficient andreliable remote communication between transmitters 17 andreceiver/stimulator unit 14 and/or 14'. While described herein asapplied to system 10, those skilled in the art will appreciate thatthese aspects of the invention can be applied to great advantage in awide variety of applications in addition to controlling the whereaboutsof animals. Moreover, while disclosed herein as utilizing a particularform of constant envelope modulation known as binary phase shift keying(BPSK), those skilled in the art will readily appreciate that theinvention can readily be implemented in forms utilizing other modulationschemes including without limitation other constant envelope modulationschemes such as pulse width modulation (PWM), minimum shift keying(MSK), and other forms of phase shift keying (PSK).

As noted above, each transmitter 17 intermittently transmits a digitallyencoded RF signal 18 at a desired carrier frequency. In the preferredembodiment, RF signal 18 is preferably transmitted in a near-fieldregime using an antenna which is electrically small and transmitting ata low RF carrier frequency, preferably one of about 10 KHz or less, suchas 8.19 KHz. This provides a number of important advantages. The fieldassociated with an RF signal of such a low frequency in a near-fieldregime is predominantly magnetic rather than electric in nature andexhibits a magnetic field whose magnitude decreases in proportion tocube of distance (r³) rather than the square (r²) of distance from itssource. This facilitates the establishment of electronic boundaries 20which are both localized and positionally well-defined. The use of sucha low RF frequency has also been found to afford high immunity to signalblockage, reflection and/or other signal propagation anomalies in areasadjacent to wholly or partially surrounded by metallic surfaces.

Referring to the circuit schematic of FIG. 2A and FIG. 2B, the structureand operation of a typical transmitter 17 will now be described.Transmitter 17 generates and transmits a digitally encoded RF signal 18at an RF carrier frequency of 8.19 KHz with power derived solely from asmall, long life lithium battery 36. Each transmitter 17 includes adrive circuit 38, a resonant circuit 40 and an isolation circuit 42interposed between the drive circuit 38 and the resonant circuit 40.Resonant circuit 40 includes a transmitting antenna 44 and preferablytakes the form of a tank circuit tuned to resonate at the RF carrierfrequency. Antenna 44 includes a ferrite core 46 upon which is woundboth a primary winding 47, terminating in a pair of leads 49 and 50, anda secondary winding 52, terminating in a pair of leads 54 and 55.Resonant circuit 40 consists further of a pair of capacitors 57 and 58and a resistor 59, all connected mutually in parallel across secondarywinding 52. For best efficiency, antenna 44 should have the highestpossible quality factor, Q, and the overall Q factor of resonant circuit40 is reduced as necessary by the loading effects of resistor 59 whoseresistance is selected to provide a Q factor yielding the minimumbandwidth necessary to accommodate the data transfer rate required by aparticular application. In the preferred embodiment, resonant circuit 40has an overall quality factor of about forty three (Q=43).

Drive circuit 38 includes an oscillator circuit 61 which serves as amaster clock. Oscillator circuit 61 includes a NAND gate, U6D, connectedto a crystal 63 by way of resistors 64 and 65 and capacitors 66 and 67as shown. Oscillator circuit 61 generates a 32.768 KHz master clocksignal which is applied to the clock (CLK) input of a twelve stageripple counter, U1. Ripple counter U1 serves as a frequency divideroperable to generate several properly timed signals required by drivercircuit 38.

Outputs Q11 and Q12 of ripple counter U1 are connected to respectiveinputs of a NAND gate U6A to provide, at output pin 3 thereof, a dutycycle control signal 70. According to an important aspect of theinvention, inactive intervals are interposed between at least some andpreferably each of the successive transmissions of RF signal 18. In thepreferred embodiment, both the duration of each transmission of RFsignal 18 and length of the inactive intervals between the intermittenttransmissions of RF signal 18 are determined by duty cycle controlsignal 70. To minimize the drain on battery 36 and thus providetransmitters 17 with long operating life, duty cycle control signal 70is timed to provide the shortest duration of individual transmissions ofRF signal 18 and to maximize the inactive intervals between successivetransmissions. In the preferred embodiment, duty cycle control signal 70also serves to deactivate as much of the circuitry of transmitter 17 aspossible during the inactive intervals between successive transmissionsof RF signal 18 thereby enhancing the efficiency at which transmitter 17converts D.C. power from battery 36 to useful radiated RF energy.

As illustrated in the timing diagram of FIG. 3, duty cycle controlsignal 70 takes the form of a periodic signal which assumes a low logiclevel for thirty-one and one-quarter milliseconds (31.25 mSec) and ahigh logic level for the remainder of its total period of one hundredtwenty-five milliseconds (125 mSec). In the preferred embodiment, the31.25 millisecond interval represents the maximum duration of each RFsignal 18 while the remainder of the period (i.e., 93.75 milliseconds)corresponds to the inactive intervals between successive transmissions.The duration of the inactive intervals between successive transmissionsof RF signal 18 is preferably maximized in order to maximize energysavings but is limited by the amount of time a particular boundary canremain inactive yet still be effective in a particular application. Thiswill depend on a variety of factors such as the location of theboundary, the maximum expected speed of the animal in the vicinity ofthe boundary and the temperament of the animal. Preferably the inactiveintervals between successive transmissions are selected to be in a rangefrom about thirty milliseconds (30 mSec) to about one hundredmilliseconds (100 mSec) or more. Indeed, in the case of certain indoorboundaries, inactive intervals of up to about five seconds (5 Sec) ormore may be suitable.

By establishing predetermined time intervals between transmissions of RFsignal 18, duty cycle control signal 70 also plays an important role inavoiding the possibility of reception of false signals which mightotherwise result in shocking animal 12 in error. As will be explainedhereinafter, receiver/stimulator verifies reception of a valid RF signal18 by requiring at least one subsequent RF signal 18 to be receivedwithin a limited time window occurring a predetermined time after aninitial reception of RF signal 18. Preferably, the width of such a timewindow only slightly exceeds the duration of transmission of each RFsignal 18 and begins at a time following reception of a prior RF signal18 which is consistent with the duration of the inactive intervalsbetween successive transmissions. If a subsequent RF signal 18 is notreceived within the predetermined time window receiver/stimulator unit14 will not allow stimulation of animal 12 to occur.

Output Q6 of ripple counter U1 oscillates between high and low logiclevels at 512 Hz and is connected to the input of pin 2 of an ExclusiveOR gate U2A which serves as a buffer and an inverter by virtue of havinga second input tied to the positive supply, +V. Output pin 3 of U2Aprovides a 512 Hz data clock signal 75. Ripple counter U1 alsogenerates, by way of its output Q2, an RF carrier signal 80 which isapplied to the input of a NAND gate U6C at pin 9 thereof. RF carriersignal 80 consists of a continuous stream of carrier pulses 82 (FIG. 3)occurring at a frequency of 8.19 KHz which corresponds as closely aspossible to the resonant frequency of resonant circuit 40. NAND gate U6Cincludes a second input at pin 8 thereof. The input at U6C pin 8 iscoupled to the output pin, pin 3, of NAND gate U6A by way of a NANDgate, U6B. NAND gate U6B has an input at pin 5 tied to the positivesupply, +V, and a second input, pin 6, connected to pin 3 of NAND gateU6A to receive duty cycle control signal 70. NAND gate U6B inverts dutycycle control signal 70 thereby delivering to the input at pin 8 of aNAND gate U6C, a signal 83 which takes the form of 31.25 millisecondpulses occurring every 125 milliseconds. NAND gate U6C logicallycombines these pulses with the RF carrier signal 80 continuously presentat its other input, U6C pin 9, in order to generate at its output, U6Cpin 10, a signal 86 in the form of successive, 31.25 millisecond burstsof 8.19 KHz RF carrier signal repeated once every 125 milliseconds.Output pin 10 of NAND gate U6C, in turn, is connected to a first inputat pin 5 of an Exclusive OR gate U2B whose operation will be explainedshortly.

In order to provide RF signal 18 with an identifiable digital code asdescribed above, the drive circuit 38 of each transmitter 17 includes acode generator 89. In the preferred embodiment, code generator 89 isimplemented using a pair of 8-stage shift registers, U3 and U5 connectedin cascade with one another. This is achieved by connecting the serialoutput, Q8 of shift register U3 to the serial input (SERIN) of shiftregister U5 and by connecting the serial output Q8 of shift register U5to the serial input (SERIN) of U3. In order to minimize drain on battery36 imposed by shift registers U3 and U5, duty cycle control signal 70 isapplied to an ENABLE input at pin 9 of both shift registers U3 and U5.Doing so allows shift registers U3 and U5 to operate and draw power onlyduring the relatively short intervals when duty cycle control signal 70is in its low logic state. Each shift register U3, U5 includes a clock(CLK) input which is tied to pin 3 of gate U2A in order to receive 512Hz data clock signal 75 which controls the rate at which code data isdelivered from code generator 89.

Shift registers U3 and U5 also each include a respective series of eightinputs, identified as P11 through P18, which are selectively connectedeither to the positive supply voltage, +V, or to circuit ground in orderto define an arbitrary 16 bit binary (four digit hexidecimal) digitalcode. In the particular transmitter 17 illustrated in FIGS. 2A and 2B,these inputs are configured to generate the hexidecimal code "F590."This is achieved by connecting inputs P18, P17, P16, P15, P13 and P11 ofU5 and inputs P18, P15 of U3 all directly to the positive supply, whileconnecting inputs P14 and P11 of U5 and inputs P17, P16, P14, P13, P12and P11 of U3 all directly to circuit ground. To facilitate assignmentand/or changing of digital codes, one or more of the inputs P11 throughP18 of shift registers U3 and/or U5 could alternatively be connected tothe positive supply, +V, by way of high resistance pull-up resistorswhile being selectively connectable to ground by way of miniatureswitches, severable links or jumper wires. Such an arrangement wouldpermit a single circuit board layout to be used to fabricatetransmitters 17 capable of being identified by any of a number ofdifferent codes. It would also allow simple and rapid assignment orchanging of codes in the field.

The operation of code generator 89 is such that each time data clocksignal 75 undergoes a logic level transition during an interval whenduty cycle control signal 70 is in a low logic state, individual binarybits of the assigned digital code are successively clocked out of theserial output Q8 of U5 as modulation data 92 as illustrated in FIG. 3beneath its corresponding hexadecimal representation 94.

In order to modulate RF signal 18 with modulation data 92, the serialoutput Q8 of shift register U1 is connected to a second input ofExclusive OR gate U2B at pin 4 thereof. As described above, Exclusive ORgate U2B includes a first input at U2B pin 5 which receives a 31.25millisecond burst of 8.19 KHz RF carrier signal once every 125milliseconds. Exclusive OR gate U2B operates as a binary phase shiftmodulator which mixes the RF carrier bursts appearing at U2B pin 5 withthe modulation data 92 presented by code generator 89 at U2B pin 4. Inso doing, Exclusive OR gate U2B induces a phase shift of one hundredeighty degrees (180°) in the RF carrier whenever the modulation data 92undergoes a logic level transition thus generating a binary phase shiftkeyed (BPSK) signal 96 at its output, U2B pin 6. BPSK signal 96 takesthe form of intermittent bursts 98 of binary phase shift-encoded carrierpulses 82 occurring nominally at the RF carrier frequency of 8.19 Khz.In accordance with duty cycle control signal 70, each burst persists forno more than 31.25 milliseconds and repeats every 125 milliseconds Theoccurrence of the phase shifts of pulses 82 is indicated schematicallyin FIG. 3 by the heavy vertical lines appearing at several points inBPSK signal 96. A portion, indicated by reference numeral 100 in FIG. 3,of a typical one of such bursts at a typical data transition isillustrated in the expanded time scale timing diagram in FIG. 4.Additional reference will now be made to FIG. 4 as the structure andoperation of the transmitter 17 circuit of FIG. 2A and FIG. 2B isfurther explained.

In order to adjust the range of RF signal 18 and thus the distancebetween each transmitter 17 and its associated electronic boundary 20,transmitter 17 includes a variable time constant differentiator circuit103. Differentiator circuit 103 takes the form of a capacitor 105 and avariable resistor 106. One side of capacitor 105 is connected to pin 6of Exclusive OR gate U2B to receive BPSK signal 96 therefrom while itsother side is connected to ground through variable resistor 106. As FIG.4 illustrates, differentiator circuit 103 differentiates each of thecarrier pulses 82 of BPSK signal 96 to generate a differentiated BPSKsignal 108 characterized by sharply peaked pulses 110 which bear a fixedphase relationship to the corresponding carrier pulses 82 of BPSK signal96. While not shown in FIG. 4, peaked pulses 110 will actually beshifted in phase by a constant amount relative to carrier pulses 82.That phase shift is immaterial however because it is constant. The peaksof differentiated BPSK signal 108 decay exponentially at a ratedetermined by the time constant of differentiator circuit 103 which canbe adjusted by means of variable resistor 106. Variable resistor 106thus serves to adjust the amount of energy contained in pulses and thus,the effective range of transmitter 17.

Differentiator circuit 103 is connected to the input of an Exclusive ORgate U2C at pin 10 thereof in order to apply differentiated BPSK signal108 thereto. Exclusive OR gate U2C includes a second input at U2C pin 9which is tied to the positive supply, +V. Exclusive OR gate U2C servesas a squaring amplifier which converts differentiated BPSK signal 108into a BPSK driving signal 112 appearing at the output pin 8 of U2C. Asshown in FIG. 4, BPSK driving signal 108 takes the form of a train ofbrief, rectangular, drive pulses 14 mutually separated by intervals 116of time. Drive pulses 114 correspond in phase to peaked pulses 10 andthus repeat substantially at the RF carrier frequency of 8.19 KHz. Byadjusting variable resistor 106, the width of drive pulses 114 can bevaried within a range from about seventy five nanoseconds (75 nSec) toabout five microseconds (5 μSec).

When BPSK driving signal 112 is applied to resonant circuit 40, itcauses resonant circuit 40, including antenna 44, to resonantly ringthereby transmitting RF signal 18. To minimize the damping effect ofeach drive pulse 114 on resonant circuit 40, the invention contemplatesthat each drive pulse 114 be as brief as practicable while stillcontaining a sufficient amount of energy to deliver the amount ofradiated RMS power required by a particular application. Widening ofdrive pulses 114 any more than is necessary tends to increase theirdamping effect on resonant circuit 40and thus decrease the efficiency atwhich transmitter 17 converts D.C. power to useful radiated RF power. Inaccordance with the invention, the duty cycle of BPSK driving signal 112should be less than about fifteen percent (15%) and preferably less thanabout five percent (5%). Most preferably, BPSK driving signal 112 has aduty cycle of less than about one percent (1%). In the preferredembodiment, for example, the width of drive pulses 114 can be adjustedusing variable resistor 106 such that the duty cycle of BPSK drivingsignal 112 ranges from about six hundredths of one percent (0.06%) toabout four point one percent (4.1%).

In order to further enhance the efficiency of transmitter 17, a furtheraspect of the invention permits drive pulses 114 to be delivered at highcurrent while minimizing any loading effects of drive circuit 38whichcould otherwise damp the ringing of resonant circuit 40. To do so, theinvention effectively electrically isolates drive circuit 38 fromresonant circuit 40 during at least a portion, and preferablysubstantially the entirety, of the intervals 116 between drive pulses114. This is done by operably interposing isolation circuit 42 betweendrive circuit 38 and resonant circuit 40.

In the preferred embodiment, isolation circuit 42 takes the form of apair of DMOS field effect transistors (FETs) 120 and 121, each havingrespective gate (G), drain (D) and source (S) terminals connectedmutually in parallel with one another as illustrated in FIG. 2B. Thegates of FETs 120 and 121 are each connected to U2C pin 8 to receiveBPSK driving signal 112. The output of isolation circuit 42 is coupledto resonant circuit 40 by connecting the common drains of FETs 120 and121 to lead 49 of the primary winding 47 of antenna 44 and to ground byway of a clamping diode 123. The common source terminals of FETs 120 and121 are connected to the positive supply, +V.

In operation, FETs 120 and 121 remain in a cutoff state until a drivepulse 114 of BPSK driving signal 112 is delivered to their gates fromthe output of Exclusive OR gate U2C. When a drive pulse 114 appears attheir gates, FETs 120 and 121 are driven rapidly into full conductionproviding a very low resistance current path from the +V supply to theprimary winding 47 of antenna 44. In doing so, isolation circuit 42allows driving pulses 114 to effectively be delivered to resonantcircuit 40 at high current with very little forward loss. Currentcontinues to be delivered to primary winding 47 until the present drivepulse 114 terminates. At that time, FETs 120 and 121 both abruptly entera cutoff state in which substantially no conduction occurs. Clampingdiode 123 serves to maintain this cutoff state during the entireinterval 116 until the next successive drive pulse 114 occurs byblocking any voltages reflected to primary winding 47 due to ringing ofthe secondary winding 52 of antenna 44. FETs 120 and 121 remain in thecutoff state during each interval 116 between drive pulses 114 so thatduring intervals 116, isolation circuit 42 presents a substantiallyinfinite impedance to resonant circuit 40 which prevents drive circuit38 from loading resonant circuit 40 during intervals 116. In sum,isolation circuit functions essentially as a current amplifier in theform of a switch which closes to deliver the drive pulses 114 of BPSKdriving signal 112 to resonant circuit 40 and which selectively opensduring intervals 116 to avoid damping the ringing of resonant circuit 40by drive circuit 48 during intervals 116.

In response to the delivery of the drive pulses 114 of BPSK drivingsignal 112 by way of isolation circuit 42, resonant circuit 40,including the secondary winding 52 of antenna 44, is caused toresonantly ring at the desired RF carrier frequency in the mannerillustrated in FIG. 4 thereby transmitting a corresponding RF signal 18.Whenever the modulation data 92 generated by code generator 89 undergoesa data transition such as one as illustrated at reference numeral 100 inFIG. 3 and more clearly by the data transition indicated in FIG. 4, BPSKsignal 96 undergoes a phase shift of one hundred eighty degrees (180°).This phase shift is reflected in differentiated BPSK signal 108, in BPSKdriving signal 112 and ultimately in RF signal 18 as indicated. It is inthis manner that the digital code provided by code generator 89 isultimately imparted to RF signal 18.

As can be seen with reference to FIG. 4, RF signal 18 does not vary inamplitude as a result of its modulation. Instead, RF signal 18 isbounded by an imaginary envelope 125 of substantially constantmagnitude. Use of such a "constant envelope" modulation scheme offersimproved reliability over amplitude modulated (AM) communications byproviding a signal which is always detectable at its maximum amplitude.Moreover, the energy efficiencies afforded by the invention are suchthat quiescent current consumption of the entire circuit of transmitter17 at its minimum range setting is only 5.5 microamperes. A typicaltransmitter 17 constructed according to the preferred embodiment iscapable of continuous operation for up to 7.7 years on a single 550milliampere-hour battery 36.

Referring now to FIG. 5 and FIG. 6, the construction of antenna 44 willnow be described in further detail. The secondary winding 52 of antenna44 consists of one thousand thirty (1030) turns of #32 AWG magnet wirewound around ferrite core 46 and terminating in secondary leads 54 and55. Ferrite core 46 may suitably comprise one such as a part number81400 available from Morely Transformer of Muncie, Ind. Ferrite core 46has a magnetic permeability of eight hundred (800), an overall length of3.1 inches and an elliptical cross section 0.5 inch wide with a maximumthickness of about 0.125 inch as illustrated in FIG. 6. Core 46 maysuitably be fabricated from material designated SPX 4803000 availablefrom National Magnetics Group of Bethlehem, Pa. or equivalent. As FIG. 5shows, the central portion of secondary winding 55 is overlaid byprimary winding 47 which consists of one hundred twenty five (125) turnsof #32 AWG magnet wire terminating in primary leads 49 and 50. Primarywinding 47 occupies the central 1.35 inches of secondary winding 52. Inthe preferred embodiment, antenna 44 has a quality factor, Q, of abouteighty (Q=80).

Table 1 below is a parts listing for the transmitter circuit of FIGS. 2Aand 2B.

                  TABLE 1                                                         ______________________________________                                        Ref-            Quan-             Manufacturer's                              erence                                                                              Description                                                                             tity    Manufacturer                                                                            Part Number                                 ______________________________________                                        106   100 Kohm  1       Bourns Inc.                                                                             3352T-104-ND                                      variable          (Riverside CA)                                              resistor                                                                44    antenna   1       see FIG. 5 & FIG. 6                                   120,  -1.5 V P  2       Zetex Inc.                                                                              BSS84                                       121   channel           (Commack NY)                                                DMOS FET                                                                U1    12 stage  1       National  CD4040BM                                          ripple            Semiconductor                                               counter           Corp. (Santa                                                                  Clara CA)                                             36    3 volt 500                                                                              1       Duracell USA                                                                            DL2450                                            mA hour           (Bethel CT)                                                 lithium                                                                       battery                                                                 123   diode,    1       Diodes Inc.                                                                             DL4148                                            DL-35, SM         (Westlake Village                                                             CA)                                                   U6A-  Quad NAND 1       Motorola, Inc.                                                                          MC14011UBD                                  U6D   gate              (Schaumburg IL)                                       U3-U5 8-stage shift                                                                           2       Motorola, Inc.                                                                          MC14021BD                                         register          (Schaumburg IL)                                       U2A-  Quad      1       Motorola, Inc.                                                                          MC74HC86D                                   U2C   Exclusive-        (Schaumburg IL)                                             OR gate                                                                 57    1000 pF,  1       Surface   MCCE102K3NR-                                      0805 Pkg.,        Mountable T1                                                10%,              Electronic                                                  capacitor         Comonents, Inc.                                                               (Austin TX)                                           67    20 pF, 0805                                                                             1       Surface   MCCE200J2NO-                                      Pkg.,             Mountable T1                                                5%,               Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           66    5 pF, 0805                                                                              1       Surface   MCCE5R1D2NO-                                      Pkg.,             Mountable T1                                                5%,               Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           105   68 pF, 0805                                                                             1       Surface   MCCE680J2NO-                                      Pkg.,             Mountable T1                                                5%,               Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           65    10 Megohm 1       ROHM Co., MCR10-EZH-JW-                                     0805 Pkg.,        LTD. (Antioch                                                                           106                                               5%, resistor      TN)                                                   59    16 Kohm,  1       ROHM Co., MCR10-EZH-JW-                                     0805 Pkg.,        LTD. (Antioch                                                                           163                                               5%, resistor      TN)                                                   64    470 Kohm, 1       ROHM Co., MCR10-EZH-JW-                                     0805 Pkg.,        LTD. (Antioch                                                                           474                                               5%, resistor      TN)                                                   58    0.01 uF,  1       NIC Components                                                                          NSPC103J50TRB2                                    0805 Pkg.,        Corp. (Amityville                                           5%, plastic       NY)                                                         capacitor                                                               63    32 KHz    1       Raltron (Miami                                                                          R38-32.768 KHz                                    Crystal           FL)                                                   ______________________________________                                    

Receiver/stimulator units 14, 14' operate to deliver an appropriateaversive stimulus in the event animals 12 or 12' encroach upon anelectronic boundary 20 or 23 defined by an RF signal 18 identified bymodulation data 92 to which a particular receiver/stimulator unit 14,14' is programmed to respond. Each receiver/stimulator unit 14, 14'includes a microprocessor based control and stimulation subsystem 130,an RF receiver 133 and a quadrature demodulator 136 which are housed ina common housing secured to a collar for affixation to animal 12, 12'.

Referring to FIG. 7, the structure and operation of the receiver 133 ofa typical receiver/stimulator unit 14 will now be described. Receiver133 as well as all other circuitry of receiver/stimulator unit 14, arepowered by a single replaceable battery 138 connected in parallel with afilter capacitor 139. Battery 138 also supplies the operating powerrequirements of quadrature demodulator 136 and control and stimulationsubsystem 130. In order to conserve battery 138, the flow of operatingcurrent from battery 138 to the remainder of the circuitry of receiver133 is controlled by means of a field effect transistor (FET) 140 whosegate (G) is connected to an output line, CTL, of control and stimulationsubsystem 130 which will be described in further detail below withreference to FIG. 9. A high resistance pull-up resistor 141 is connectedbetween the gate and source terminals of FET 140. FET 140 also includesa drain (D) terminal which is bypassed to ground through a capacitor142. The drain (D) of FET is connected to a positive supply rail 144 byway of a current limiting resistor 145 by way of a line 147 bypassed toground through a capacitor 148. Positive supply rail 144 is itselfbypassed to ground by a pair of capacitors 149 and 150. As indicated inFIG. 7, line 147 traverses an RF shield 152 which takes the form of anenclosure formed of copper sheet which encloses the circuitry indicatedand serves to shield it from noise.

The circuitry of receiver 133 described thus far operates to selectivelycontrol the flow of electrical power from battery 138 to positive supplyrail 144. When the CTL line from control/stimulation subsystem 130 is ina low logic state, FET 140 is driven into conduction to provide a pathfor delivering current from battery 138 to positive supply rail 144 byway of resistor 145. When control and stimulation subsystem 130 drivesits CTL line to a high logic state, FET 140 is rapidly cut off therebyblocking the flow of current from battery 138 to positive supply rail144. In the event of a failure in the control and stimulation subsystem130, pull-up resistor 141 serves to hold FET 140 in a cutoff state inorder to conserve battery 138.

In order to intercept an RF signal 18 when animal 12 encroaches uponeither an electronic boundary 20 associated with a transmitter 17 or thewire loop boundary 23 emitted by wire loop 22, receiver 133 includes anantenna 155. Antenna 155 consists of a pair of mutually orthogonallypositioned, twelve millihenry (12 mH) inductors 156 and 157. Inductors156 and 157 are connected electrically in parallel both with one anotherand with a series of parallel capacitors 158, 159 and 160 and a resistor161 to form a resonant front-end filter circuit 164. Capacitors 158, 159and 160 are selected to resonate with inductors 156 and 157 at the RFcarrier frequency of 8.19 KHz. Resistor 161 loads the resonant circuitin order to lower its quality factor, Q, to a value which widens itsbandwidth sufficiently to pass the frequency components of interestassociated with the modulation data 92 component of the RF signal 18intercepted by antenna 155.

Front end filter circuit 164 is connected to coupling capacitor 166which delivers the RF signal 18 intercepted by antenna 155 to a class A,common emitter amplifier stage consisting of an NPN type bipolarjunction transistor 169 connected in the conventional manner to thepositive supply rail 144 and circuit ground by way of associated biasingresistors 171, 172, 173 and 174 as well as bypass capacitors 175 and 176as shown. This first stage of amplification imparts a voltage gain ofabout twenty-nine decibels (29 dB) to the RF signal 18 presented byfront end filter circuit 164.

The collector of transistor 169 is connected by way of an a.c. couplingcapacitor 180 to a second class A, common emitter amplifier stageconsisting of an NPN type bipolar junction transistor 183, associatedbiasing resistors 185, 186, 187 and 188 as well as bypass capacitors 189and 190. This second stage of amplification imparts an additionalvoltage gain of about twenty nine decibels (29 dB) to the RF signal 18.In order to reduce the out-of-band noise which might otherwise cause afalse indication that an appropriately encoded RF signal 18 has beenreceived, the emitter of transistor 183 is connected to a resonantbandpass filter 191. Filter 191 consists of an inductor 192, a capacitor193 and a capacitor 194 all of which are connected mutually in parallelbetween the emitter of transistor 183 and positive supply rail 144.Filter 191 resonates at a center frequency of 8.19 KHz and ischaracterized by a quality factor, Q, of about four point four (Q=4.4)to provide a bandwidth of about 1.86 KHz.

Having been stripped of out-of-band noise by the action of bandpassfilter 191, RF signal 18 is amplified further. This is achieved byconnecting the collector of transistor 183 by way of an a.c. couplingcapacitor 197 to a third, and subsequently to a fourth, class A commonemitter amplifier stage, each of which provides an additional voltagegain of about twenty-nine decibels (29 dB). The third stage consists ofan NPN type bipolar junction transistor 200 connected conventionally tosupply rail 144 and ground by way of biasing resistors 203, 204, 205 and206 and bypass capacitor 207. The fourth amplification stage isconnected to the collector of amplifier 200 by way of an a.c. couplingcapacitor 210. This fourth stage consists of an NPN type bipolarjunction transistor 212, connected in the conventional manner to supplyrail 144 and ground by way of associated biasing resistors 215, 216, 217and 218 and bypass capacitors 219 and 220 as shown.

A fully amplified rendition of the received RF signal 18 appears at thecollector of transistor 212. As a further precaution against anerroneous determination that a properly encoded RF signal 18 has beenreceived, a D.C. offset voltage of about 0.53 Volts is imparted to thissignal. This is achieved by connecting the collector of transistor 212through an a.c. coupling capacitor 222 to the center of a voltagedivider formed by a pair of resistors 224 and 225 connected in serieswith one another between positive supply rail 144 and circuit ground.The junction between resistors 224 and 225 is in turn connected toquadrature demodulator 136 by way of a current-limiting resistor 227.Referring to FIG. 8 the structure and operation of quadraturedemodulator 136 will now be described.

Quadrature demodulator 136 is driven by a clock circuit 229 whichgenerates a pair of 8.19 KHz signals 230 and 231 which are ninetydegrees (90°) out of phase with one another. Clock circuit 229 includesa crystal 233 connected across an inverter U31 by way of a networkconsisting of resistors 236 and 237 and a capacitor 238 to form anoscillator which generates a free running 32.768 KHz signal at theoutput of U31A. The output of U31A is connected to the clock (CLK) inputof a D-type flip-flop U33A configured as a frequency divider. Flip-flopU33A has an output, Q at pin 1 thereof which provides a first 16.384 KHzsignal. Flip-flop U33A has a second output, Q, at pin 2 thereof whichprovides a second 16.384 KHz signal ninety degrees (90°) out of phasewith aforementioned first 16.384 KHz signal. The output Q at pin 1 offlip-flop U33A is connected to the clock (CLK) input at pin 3 of asecond flip-flop, U33B which is likewise configured as a frequencydivider. Second flip-flop U33A includes an output Q at pin 13 thereofwhich provides one of the aforementioned 8.19 KHz signals 230 whichserves as a phase reference. In a similar fashion, the output Q at pin 2of flip-flop U33A is connected to the clock (CLK) input at pin 3 of athird flip-flop, U35A, likewise configured as a frequency divider. Thirdflip-flop U35A includes an output Q at pin 1 thereof which providesquadrature 8.19 KHz signal 231. Pin 13 of second flip-flop U33B isconnected to pin 2 at the input of a first Exclusive OR gate, U36A todeliver signal 230 thereto. Pin 1 of third flip-flop U35A is similarlyconnected to a corresponding input at pin 6 of a second Exclusive ORgate U36B to apply quadrature signal 231 thereto.

An inverter U31F includes an input which is connected to the resistor227 of FIG. 7 to receive an amplified and slightly D.C. offset renditionof RF signal 18 therefrom. Inverter U31F serves as a hard limiter whichclips the peaks from the amplified RF signal 18 to generate a clippedBPSK signal 264. The portions clipped away include any noisy portionsthereof raised beyond the clipping threshold by virtue of the D.C.offset applied by the action of resistors 224 and 225 of FIG. 7. Theoutput of inverter U31F is connected to the input at pin 1 of firstExclusive OR gate U36A as well as to the input at pin 5 of secondExclusive OR gate U36B. Exclusive OR gates U36A and U36B serve to theclipped BPSK signal delivered at the output of U1F with phase referencesignal 230 and quadrature signal 231.The outputs of Exclusive OR gatesU36A and U36B are connected to respective low pass filters 267 and 268which respectively consist of a resistor 270 and a capacitor 271 and aresistor 272 and a capacitor 273. Low pass filters 267 and 268 each havea -3 dB cutoff frequency of about 1 KHz thereby serving to recover fromthe clipped BPSK signal 26 both a code data signal 275 and a quadraturecode data signal 276 which are buffered by respective inverters U31B andU31C connected to the output sides of low pass filters 267 and 26,respectively. The outputs of inverter U31B and U31C are connected torespective in-phase and quadrature (Q) inputs of the control andstimulation subsystem 130. Since the true phase of the clipped BPSKsignal 264 is not known, either code data signal 275 or quadrature datasignal 276 will faithfully represent the modulation data signal 92 shownin FIG. 3 which identifies a particular transmitter 17 or, as will beexplained hereinafter, wire loop 22.

Referring now to FIG. 9, control and stimulation subsystem 130 ispowered entirely from battery 138 which is connected to a positivesupply rail 283 by way of a current-limiting resistor 284. Power supplyrail 283 is bypassed to circuit ground by way of a pair of capacitors282 and 285. Subsystem 130 includes a programmed microprocessor 281which receives operating power by virtue of connections to positivesupply rail 283 and to circuit ground at pins designated VDD and VSS,respectively. Microprocessor 281 internally includes sufficient programmemory and data memory as well as an arithmetic and logic unit,input/output facilities, a data bus, an address bus and otherconventional timing and control facilities. Microprocessor 281 operatesat a clock speed of 400 KHz established by a crystal 286 connected to anexternal oscillator input (OSC 2) at pin 15 of microprocessor 281 and tocircuit ground by way of a pair of capacitors 287 and 288. A real timeclock/counter (RTCC) input at pin 15 of the preferred microprocessor 281identified in Table 2 below is not used and is tied to ground.Microprocessor 281 is programmed to have stored therein an operatingprogram and certain data as set forth in detail in the relevant portionsof the Software Appendix referred to above in order to control theoperation of subsystem 130 in accordance with various inputs in themanner to be described. For that purpose, microprocessor furtherincludes a plurality of bidirectional input/output (I/O) portsidentified as RA0 through RA3 and RB0 through RB7. Individual ones ofthese ports are configured either as input ports or as output portsaccording to the stored operating program.

In order to allow microprocessor 281 to be programmed serially in thefield, subsystem 130 includes programming port 291 physically accessiblethrough the battery compartment of receiver/stimulator unit 14.Programming port 291 includes a series of three pins 292, 293 and 294which may be used to program microprocessor 281 in the manner more fullydescribed in programming specification DS30189 using hardware describedin publication DS00589A, both of which are available from MicrochipIncorporated of Dallas, Tex. and which are expressly incorporated hereinby reference in their entirety to form part of the present disclosure.Pin 291 is connected to positive supply rail 283 by way of a pull-upresistor 296 as well as to reset pin 4 of microprocessor 281 while pins293 and 294 are connected respectively to I/O ports RB6 at pin 6 and RB7at pin 7 of microprocessor 281.

Receiver/stimulator units 14, 14' conserve battery 138 by selectivelyshutting down circuits which are non-essential at times when animal 12is not moving. For that purpose, subsystem 130 is provided with a highlysensitive, single pole, single throw motion switch 299. One side ofmotion switch 299 is connected to circuit ground while its other side isconnected to positive supply rail 283 by way of a voltage dividerconsisting of a resistor 300 and a resistor 301. To enablemicroprocessor 281 to read the state of motion switch 299, resistors 300and 301 are connected at their common junction to an input port at RB5at pin 11 of microprocessor 281. Motion switch 299 is mounted on theprinted circuit board associated with subsystem 130 and opens and closesfrequently when animal 12 is moving about. To selectively disable thebattery conserving function associated with motion switch 299 forconvenience in troubleshooting or testing system 10, one can install ajumper 304 to selectively tie I/O port RB0 at pin 6 to circuit ground.Port RB0 at pin 6 of microprocessor 281 is configured as an input portand is connected to positive supply rail 283 by way of a pull-upresistor 304 which acts to hold port RB0 at a high logic state at alltimes when jumper 304 is not installed, thereby enabling the batteryconserving function associated with motion switch 299. That function isimplemented using I/O port RA3 at pin 2 of microprocessor 281 which isconfigured as an output port and is connected by way of a resistor 306to the control (CTL) input at the gate of FET 140 shown in FIG. 7.

When no jumper 304 is installed and animal 12 is determined bymicroprocessor 281 to be moving based on the inputs received from motionswitch 299 by way of I/O port RB5, microprocessor 281 drives I/O portRA3 at pin 2 to a low logic level state thereby delivering operatingpower to receiver 133. Conversely, when the input received from motionswitch 299 indicates animal 12 is not moving, there is no need forsubsystem 130 to continue monitoring for encroachment of a boundary 20or 23 as long as animal 12 remains not in motion. In that event,microprocessor 281 drive I/O port RA3 at pin 2 to a high logic level tocut off power to portions of circuitry of receiver 133.

To monitor the condition of battery 138, subsystem 130 includes avoltage detector 307 connected to positive supply rail 283 and circuitground at pins 2 and 3 of detector 307, respectively. Voltage detector307 includes an output pin, pin 1, which is connected to I/O port RB4 atpin 10 of microprocessor 283 which is configured as an input port. Whenpower supply rail 283 drops to a voltage below a threshold of about 2.35volts, pin 1 of voltage detector 307 undergoes a logic level transitionwhich microprocessor 281 senses by sampling the signal at port RB4. Toavoid the possibility of unpredictable operation in the event positivesupply rail 283 drops below a second predetermined threshold voltage, abrownout protection circuit (not shown) may optionally be provided toreset microprocessor 281. Such a brownout protection circuit isdescribed in detail in publication DS30081B available from MicrochipIncorporated of Dallas, Tex. which is expressly incorporated herein byreference in its entirety to form part of the present disclosure. Avisible indication that battery 138 is in need of being replaced,subsystem 130 is provided by a light-emitting diode (LED) 308 whosecathode is connected to circuit ground and whose anode is connected byway of a resistor 309 to port RA2 at pin 2 of microprocessor 281. PortRA2 is configured as an output port which remains in a low logic stateduring normal operation. In the event the voltage on positive supplyrail 283 drops below the threshold determined by voltage detector 307,voltage detector 307 applies a signal to the input port RB4 ofmicroprocessor 281. In response to that signal, the operating programstored in microprocessor 281 toggles output port RA2 to blink LED 308which is mounted so as to be visible exteriorly of receiver/stimulatorunit 14.

Control/stimulation subsystem 130 provides for the administration ofvarious forms of aversive stimulation to an animal 12 in the eventmicroprocessor 281 determines that an RF signal 18 satisfying one ormore predetermined conditions has been received by receiver 133. Inorder to permit such a determination to be made, the outputs ofinverters U31B and U31C shown in FIG. 8 are connected respectively toI/O ports RB2 and RB3 of microprocessor 281 at pins 8 and 9 thereof,respectively. Ports RB2 and RB3 are configured as input ports to enablemicroprocessor 281 to read code data signal 275 and quadrature code datasignal 276, respectively. The manner in which microprocessor 281determines whether an RF signal 18 satisfying predetermined conditionshas been received will be explained in further detail below withreference to the flowchart of FIG. 15.

In the preferred embodiment, stimulation can take the form either solelyof a tone audible to animal 12 or as an audible tone and administeredsubstantially simultaneously with a harmless electrical shock. Theappropriate form and intensity of stimulation can be selected inaccordance with the size and/or temperament of the animal 12 to becontrolled. For example, a larger and/or more aggressive animal 12 canbe administered a more intense ("High") shock of about five kilovolts (5KV) in magnitude. Such "High" shocks preferably take the form of athirty to ninety millisecond burst of individual pulses about sixhundred microseconds (600 μSec) each in duration. A smaller and/or lessaggressive animal 12' can be administered only an audible tone or anaudible tone combined with a less intense ("Low") shock of about twopoint five (2.5) kilovolts (2.5 KV) in magnitude. Such shocks preferablytake the form of a series of individual pulses of about six hundredmicroseconds (600 μSec) each in duration. The form and intensity ofstimulation is selectable in the field according to the position of athree position selector switch 310 having a movable wiper 314 one end ofwhich connected permanently to circuit ground. Selector switch 310further includes a series of terminals 311, 312 and 313. Terminal 311 isconnected by way of a resistor 305 to I/O port RB6 at pin 12 ofmicroprocessor 281. Port RB6 is configured as an input port and isconnected to positive supply rail 283 by way of a pull-up resistor 302which serves to maintain port RB6 at a high logic level except whenwiper 314 is moved into contact with terminal 311. When wiper 314 ismoved into contact with terminal 311, the voltage at port RB6 ofmicroprocessor 281 drops to a low logic level. In a similar manner,terminal 313 of selector switch 310 is connected by way of a resistor297 to I/O port RB7 at pin 6 of microprocessor 281. Port RB7 is alsoconfigured as an input port and is connected to positive supply rail 283by way of a pull-up resistor 298 (R28). Resistor 298 serves to maintainthe voltage at port RB7 at a high logic level except when it is droppedto a low logic level by moving the wiper 314 of selector switch 310 intocontact with terminal 313. Terminal 312 of switch 310 is open.Positioning of the wiper 314 of selector switch 310 at terminal 312 thusresults in input ports RB6 and RB7 both remaining in a high logic levelstate.

The stored operating program causes microprocessor 281 to read theposition of selector switch 310 to determine the appropriate form andintensity of stimulation to be delivered. To do so, microprocessor 281reads the logic level states of inputs RB6 and RB7. If RB6 is determinedto be in a high logic level state, the wiper 314 of selector switch 310is determined to be in contact with terminal 310. Microprocessor 281 isthereby instructed to administer stimuli in the form of both an audibletone and a "High" intensity electrical shock. When the wiper 314 ofselector switch 310 is in contact with terminal 313, port RB7 isdetermined to be in a low logic level state. This instructsmicroprocessor 281 that stimuli in the form of an audible tone and a"Low" intensity electrical shock are to be delivered. Whenmicroprocessor 281 determines that input ports RB6 and RB7 both remainat a high logic level, microprocessor 281 is instructed to administerstimulation only in the form of an audible tone. For administeringelectrical and audible stimuli, subsystem 130 includes both an audiotransducer 315 and a pair of mutually spaced electrodes 316 and 317.

Electrodes 316 and 317 preferably take the form of stainless steel pinsabout three quarters to about one inch in length and about one eighth ofan inch in diameter. Each electrode 316, 317 has a respective distal end318, 319 which is blunted to minimize discomfort and avoid irritation tothe skin of animal 12. Each electrode 316 and 317 has a proximal end320, 321 which is preferably provided with a hexagonal head 322, 323terminating in male threaded portion 324, 325 engageable with a threadedfemale socket 326, 327. In this way, sets of interchangeable electrodes316 and 317 of different lengths accommodating animals having coats ofeither short, long and medium length hair can be provided. Electrodes316 and 317 are mounted to protrude normally from the surface of aninjection molded housing mechanically affixed to animal collar 15. Thishousing (not shown) includes a user accessible battery compartment toaccommodate battery 138 and encloses all the remaining components ofreceiver/stimulator 14 including those of control and stimulationsubsystem 130 as well as receiver 133 and quadrature demodulator 136.Collar 15 holds electrodes 316 and 317 in sufficiently close contactwith the skin of animal 12 to effectively administer electrical shockstimulation.

High voltage electrical shock pulses are generated by a step-uptransformer 331 having a primary winding 332, a magnetic core 333 and asecondary winding 334. In the preferred embodiment, the primary tosecondary turns ratio of transformer 331 is about 100:1. Secondarywinding 334 is connected electrically in parallel across female sockets326 and 327 in order to deliver high voltage shocks to animal 12 by wayof electrodes 316 and 317. One side of primary winding 332 is connecteddirectly to the positive side of battery 138 while the opposite side ofprimary winding 332 is connected to the source terminal of a powerMOSFET 336 the drain terminal of which is connected to circuit ground.In order to allow microprocessor 281 to selectively control the flow ofcurrent through primary winding 332, MOSFET 336 is connected at its gateterminal to I/O port RA0 at pin 17 of microprocessor 281 by way of aresistor 339. Port RA0 of microprocessor 281 is configured as an outputport.

When microprocessor 281 determines a shock is to be administered,mutually spaced bursts of appropriately timed, logic level, shock pulses341 are generated by microprocessor 281 at output port RA0 asillustrated in FIG. 10. Each shock pulse 341 drives MOSFET 336 intoconduction delivering a corresponding pulse of high current to primarywinding 332 which drives transformer 331 into saturation. When eachshock pulse 341 terminates, MOSFET 336 is rapidly cut off, abruptlyterminating the flow of current through primary winding 332 resulting ingeneration of a high kickback voltage across secondary winding 334 andhence, electrodes 316 and 317. The duration of each shock pulse 341, theintervals between pulses 341, the duration of each burst of shock pulses341 and the intervals between such bursts are all determined bymicroprocessor 281 under program control.

For generating aversive audio stimuli, subsystem 130 includes an audiotransducer 314 is connected electrically in series to circuit ground byway of a driver circuit 347 which consist of a pair of FET's 349 and 350connected mutually in parallel with one another as shown. The drains ofFET's 349 and 350 are each connected to audio transducer 314 while theirsource terminals are each connected to the positive side of battery 138.The gates of FET's 349 and 350 are connected directly to one another andare each connected to I/O port RA1 of microprocessor 281 by way of aresistor 351. Port RA1 is configured as an output port which is normallymaintained at a high logic level state by microprocessor 281 and pull-upresistor 352 connected between I/O port RA1 and positive supply rail283. This holds FET's 349 and 350 in a cutoff state preventing audiotransducer 314 from sounding. When microprocessor 281 determines that anaudio tone is to be delivered, microprocessor 281 generates an audiodrive signal 353 by toggling output port RA1 at an audio rate, such as3.2 KHz, in thirty to ninety millisecond bursts at mutually spacedintervals of about thirty milliseconds.

A parts listing for the circuitry of receiver/stimulator 14 includingthe receiver 133 of FIG. 7, the quadrature demodulator 136 of FIG. 8 andthe control and stimulation subsystem 130 of FIG. 9, is set forth belowin Table 2.

                  TABLE 2                                                         ______________________________________                                        Ref-            Quan-             Manufacturer's                              erence                                                                              Description                                                                             tity    Manufacturer                                                                            Part Number                                 ______________________________________                                        331   transformer                                                                             1       Microcomp Inc.                                                                          129-3141-EA                                                         (Beaverton OR)                                        192   22 mH     1       J W Miller                                                                              9250-226                                          inductor          Magnetics                                                                     (Gardenia CA)                                         140,  P channel 3       Zetex Inc.                                                                              BSS84ZX                                     349,  JFET              (Commack NY)                                          350                                                                           138   1300 mAH  1       Duracell USA                                                                            DL123A                                            battery           (Bethel CT)                                           331   DPDT      1                 GT13MABKE                                         switch                                                                  308   Light     1       Panasonic LN28RP                                            emitting          industrial Co.                                              diode             (Div. of                                                                      Matsushita                                                                    Electric Corp. of                                                             America)                                                                      (Secaucus NY)                                         U33A, Dual D-type                                                                             2       Motorola, Inc.                                                                          MC14013BCD                                  U33B, flip-flop         (Schaumburg IL)                                       U35A                                                                          U36A, Exclusive 1       Motorola, Inc.                                                                          MC14077BCD                                  U36B  NOR gate          (Schaumburg IL)                                       U31A- HEX       1       Motorola, Inc.                                                                          MC14106BCD                                  U31F  Schmitt-          (Schaumburg IL)                                             trig inverter                                                           238   10 pF, 603                                                                              1       Surface   MCCE100D1NO-                                      Pkg., .5%,        Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           189,  100 pF, 603                                                                             2       Surface   MCCE101J1NO-                                220   Pkg., 5%,         Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           180,  1000 pF, 603                                                                            4       Surface   MCCE102K1NR-                                197,  Pkg., 10%,        Mountable T1                                          210,  capacitor         Electronic                                            222                     Components, Inc.                                                              (Austin TX)                                           166   .01 uF, 603                                                                             1       Surface   MCCE103K1NR-                                      Pkg., 10%,        Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           149,  .1 uF, 603                                                                              8       Surface   MCCE104Z1NV-                                219,  Pkg., 5%,         Mountable T1                                          207,  capacitor         Electronic                                            190,                    Components, Inc.                                      285,                    (Austin TX)                                           152,                                                                          142                                                                           287,  220 pF, 603                                                                             2       Surface   MCCE221J1NO-                                288   Pkg., 5%,         Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           176   33 pF, 603                                                                              1       Surface   MCCE330J1NO-                                      Pkg., 5%,         Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           271,  560 pF, 603                                                                             2       Surface   MCCE561J1NR-                                273   Pkg., 5%,         Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           194   6800 pF, 603                                                                            1       Surface   MCCE682J1NR-                                      Pkg., 5%,         Mountable T1                                                capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           236   10 Meg    1       Surface   MCR03-EZH-J-                                      ohm,              Mountable 106                                               805 Pkg.,         Electronic                                                  5%, resistor      Components, Inc.                                                              (Austin TX)                                           145   100 ohm,  1       ROHM Co., MCR03-EZH-JW-                                     603 Pkg.,         LTD. (Antioch                                                                           101                                               5%, resistor      TN)                                                   309   2.7 Kohm, 1       ROHM Co., MCR03-EZH-JW-                                     603 Pkg.,         LTD. (Antioch                                                                           272                                               5%, resistor      TN)                                                   284   51 ohm, 603                                                                             1       ROHM Co., MCR03-EZH-JW-                                     Pkg., 5%,         LTD. (Antioch                                                                           510                                               resistor          TN)                                                   169,  Low noise 4       ROHM Co., MMST5089                                    183,  signal            LTD. (Antioch                                         200,  transistor        TN)                                                   212                                                                           315   3.2 KHz   1       Star Micronics                                                                          MUT-01A                                           piezo             Inc./OEM                                                    transducer        (Piscataway NJ)                                       304   Jumper    1       N/A       N/A                                         193,  .01 uF, 1206                                                                            2       NIC Components                                                                          NSPC103J50TRB2                              159   Pkg., 5%,         Corp. (Amityville                                           plastic           NY)                                                         capacitor                                                               160   .01 uf, 1206                                                                            1       NIC Components                                                                          NSPC103J50TRB2                                    Pkg., 5%          Corp. (Amityville                                           (select in        NY)                                                         test)                                                                   158   0.047 uf, 1       NIC Components                                                                          NSPC473J16TR83                                    1206 Pkg.,        Corp. (Amityville                                           5%, plastic       NY)                                                         capacitor                                                               156,  12 mH     2       Coilcraft Inc.                                                                          PCH-27-126                                  157   Inductor          (Cary IL)                                             281   8 bit CMOS                                                                              1       Microchip PIC16LC84-                                        micro-            Technology, Inc.                                                                        XT/SO                                             processor         (Chandler AZ)                                         233   32.768 KHz                                                                              1       Raltron (Miami                                                                          R38-32.768 KHz                                    crystal           FL)                                                   172,  100 Kohm, 5       Surface   RC73L2X100KOH                               186,  603 Pkg.,         Mountable MJT                                         204,  5%, resistor      Electronic                                            216,                    Components, Inc.                                      227                     (Austin TX)                                           352   10 Kohm,  1       Surface   RC73L2X10KOH                                      603 Pkg.,         Mountable MJT                                               5%, resistor      Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           171,  180 Kohm, 4       Surface   RC73L2X180KOH                               185,  603 Pkg.,         Mountable MJT                                         203,  5%, resistor      Electronic                                            215,                    Components, Inc.                                                              (Austin TX)                                           296,  1 Kohm, 603                                                                             7       Surface   RC73L2X1KOHM                                300,  Pkg., 5%,         Mountable JT                                          351,  resistor          Electronic                                            339,                    Components, Inc.                                      305,                    (Austin TX)                                           297,                                                                          306                                                                           141   1 Meg ohm,                                                                              1       Surface   RC73L2X1MOH                                       603 Pkg.,         Mountable MJT                                               5%,               Electronic                                                  resistor          Components, Inc.                                                              (Austin TX)                                           217,  20 Kohm,  4       Surface   RC73L2X20KOH                                205,  603 Pkg.,         Mountable MJT                                         187,  5%, resistor      Electronic                                            173                     Components, Inc.                                                              (Austin TX)                                           270,  270 Kohm, 2       Surface   RC73L2X270KOH                               272   603 Pkg.,         Mountable MJT                                               5%, resistor      Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           161   3.3 Kohm, 1       Surface   RC73L2X3.3KOH                                     603 Pkg.,         Mountable MJT                                               5%, resistor      Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           237,  470 Kohm, 5       Surface   RC73L2X470KOH                               304,  603 Pkg,          Mountable MJT                                         301,  5%, resistor      Electronic                                            302,                    Components, Inc.                                      298                     (Austin TX)                                           174,  7.5 Kohm, 4       Surface   RC73L2X7.5KOH                               188,  603 Pkg.,         Mountable MJT                                         206,  5%, resistor      Electronic                                            218                     Components, Inc.                                                              (Austin TX)                                           224   130 kohm, 1       Surface   RC73M2X130KO                                      603 Pkg.,         Mountable HMJT                                              5%, resistor      Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           225   270 kohm, 1       Surface   RC73M2X270KO                                      603 Pkg.,         Mountable HMJT                                              5%, resistor      Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           336   Power     1       Harris    RFD12N06RLE                                       Mosfet            Semiconductors                                                                (Melbourne FL)                                        307   2.35 V    1                 S-8052ALO-LG-S                              299   Motion    1       Sensormatic                                                 switch            Electronics Corp.                                                             (Deerfield Beach                                                              FL)                                                   139   100 uF SMD                                                                              1       Surface   TCC100M4D                                         D size            Mountable                                                   tantalum          Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           148,  33 uF SMD 2       Surface   TCC33M4C                                    282   C size            Mountable                                                   tantalum          Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           286   Crystal   1       Toko American                                                                           TK9940                                            C.sub.load =      Inc. (Mount                                                 12 pF             Prospect IL)                                          ______________________________________                                    

As FIG. 10 further illustrates, system 10 also permits aversivestimulation to be varied depending on the location of the boundary beingencroached upon. The code generator 89 of each transmitter 17 can beconfigured as explained above to transmit an RF signal 18 bearing anyone of nearly sixty five thousand unique digital codes. As will beexplained shortly, a unique code can also be assigned to the RF signal18 emitted by wire loop 22. By assigning different codes to some or allof the transmitters 17 as well as the wire loop 22, the wire loopboundary 23 can be distinguished from any of the electronic boundaries20 associated with transmitters 17. In addition, groups and/orindividual ones of boundaries 20 can also be distinguished from oneanother as may be desired. The operating program associated withmicroprocessor 281 determines which particular one of a plurality ofpossible digital codes is represented by the code signal 275 andquadrature code signal 276 applied to input ports RB2 and RB3,respectively. Microprocessor 281 then drives output ports RA0 and/or RA1in a particular preprogrammed manner which the operating programdetermines corresponds to a particular one of such codes. For example,it may be desired to deliver a stronger aversive stimulus in response toan encroachment of the perimeter wire loop boundary 23. As FIG. 10illustrates, microprocessor 281 achieves this by delivering shock pulses341 at port RA0 at a repetition rate of one shock pulse 341 about everyseven point five milliseconds (7.5 mSec) in the event of encroachment ofwire loop boundary 23. In contrast, shock pulses 341 are delivered atintervals of twenty two point five milliseconds (22.5 mSec) in the eventof encroachment of an electronic boundary 23 associated with atransmitter 17. As FIG. 10 further illustrates, the preferred embodimentof system 10 delivers audio stimulation in ninety millisecond (90 mSec)bursts when an electronic boundary 20 established by a transmitter 17 isencroached upon but in only thirty millisecond (30 mSec) bursts whenwire loop boundary 23 is encroached upon. Preferably, such audio burstsare loud; on the order of about eighty decibels (80 dBA). Otheralternatives are also possible. For example, to avoid disturbing thehuman occupants of dwelling 13, microprocessor 281 can be programmed toprovide audio stimulation of a form less noticeable to humans uponrecognition of a digital code corresponding to one of the transmitters17 located inside dwelling 13 such as those located at the foot ofstairway 29 and/or the entrance to room 30. This can be readily achievedby programming microprocessor 281 to provide an audio drive signal 353consisting of fewer and/or shorter bursts and/or one at an elevatedfrequency inaudible to humans but capable of being heard by animal 12'.

Microprocessor 281 is preferably programmed to strictly limit the amountof time any form of stimulation can be administered pursuant to anysingle boundary encroachment episode or series of episodes occurringvery closely in time to one another. This can readily be done using atimer implemented in software or a counter which limits the number oftimes a stimulation loop is executed. This will ensure that the animal12 will not be traumatized if for any reason the animal does not or isunable to promptly withdraw from a boundary 20 or 23 to terminate allstimuli by the animal's own volition. Microprocessor 281 is alsopreferably programmed to terminate all stimuli substantially immediatelyupon the withdrawal of the animal from a boundary 20 or 23. Doing so notonly serves to minimize any discomfort to animal 12, but also helpsavoid the possibility that the animal 12 will associate the stimuluswith a behavior or condition other than its encroachment of the boundary20 or 23. Before explaining the operation of system 10, and inparticular the programming of microprocessor 281 in further detail, thestructure and operation of wire loop unit 21 will now be described withreference to FIG. 11 and FIG. 12.

Wire loop unit 21 includes a pair of five-way binding posts 360 and 361connectable to opposite ends of wire loop 22 as shown. Wire loop unit 21derives its normal operating power from an A.C. power outlet (not shown)by way of a conventional step-down transformer (not shown) whichdelivers 6 volts A.C. by way of a female plug connector (not shown)connectable to a mating male input jack 366. Jack 366 is connected to afull wave rectifier 368 by way of a one-half half ampere resettable fuse369. The D.C. output side of rectifier 368 establishes an unregulated 6volt D.C. supply rail 370 which is connected in parallel with a filtercapacitor 371 and a six volt battery 372. Battery 372 which may suitablycomprise a six volt gel cell type battery serves as a backup powersource capable of operating wire loop unit 21 for sustained periods oftime in the event of an A.C. power outage. The unregulated six volt D.C.supply rail 370 is connected in turn to the input of a five volt,voltage regulator 381. The output side of five volt regulator 381 isparalleled with a filter capacitor 386 to establish a regulated fivevolt D.C. supply rail 388.

A crystal 395 connected to a NAND gate U43A by way of a pair ofresistors 397, 398 and a pair of capacitors 399, 400 generates a 32.768KHz signal at the output of NAND gate U43A. The output of NAND gate U43Ais connected to the input at pin 10 of a twelve stage ripple counter 402as well as to the input at pin 8 of a NAND gate U43C and to the externaloscillator input, OSC2 of a programmed microprocessor 404. Ripplecounter 403 operates as a frequency divider and includes an output, Q2at pin 7, which provides a signal at the RF carrier frequency of 8.19KHz. Ripple counter also includes an output, Q11 at pin 15 whichgenerates a signal which oscillates at about 16 Hz and an output Q12 atpin 1 which oscillates at about 8 Hz.

Microprocessor 404 internally includes sufficient program memory anddata memory as well as an arithmetic and logic unit, input/outputfacilities, a data bus, an address bus and other conventional timing andcontrol facilities. Microprocessor 404 is connected between supply rail388 and circuit ground by way of a pair of power supply inputsidentified as VDD and VSS, respectively and includes a reset input atpin 4 which is tied to supply rail 388 by way of a resistor 405.Microprocessor 404 further includes a real time clock/counter (RTCC)input at pin 3 which is unused and is connected to circuit ground.Microprocessor 404 is programmed in the manner set forth in relevantportions of the Software Appendix and includes a set of fourbidirectional input/output (I/O) ports located at pins 17, 14, 1 and 2which are identified as ports RA0, RA1, RA2 and RA3, respectively. PortsRA0, RA1, RA2 and RA3 are each configured as input ports. Input port RA0is connected to supply rail 388 by way of a pull-up resistor 406 and isalso selectively connectable to circuit ground by way of switch 407which forms part of a manually operable, quad, dual-inline-packaged(DIP) switch unit 408. In a similar fashion, output ports RA1, RA2 andRA3 of microprocessor 404 are connected to supply rail 388 by way ofrespective resistors 409, 410 and 411 and are selectively connectable tocircuit ground by way of switches 412, 413 and 414, respectively.Microprocessor 404 also includes a pair of bidirectional input/output(I/O) ports configured as output ports: RB1 at pin 7 and RB2 at pin 8.Port RB1 is connected to a reset (RST) input at pin 11 of ripple counter403. Output port RB2 is connected to an input pin, pin 1, of an inverterU42A as well as to the control input (E) at pin 5 of an analog switchU48A.

The positions of switches 407, 412, 413 and 414 of switch unit 408control the logic level of the voltages at input ports RA0, RA1, RA2 andRA3. By reading states of those inputs, the operating program storedwithin microprocessor 404 determines which one of up to sixteenpreprogrammed, sixteen bit (four digit hexidecimal) codes microprocessor404 will repetitively present at output its port RB2. Each of thesecodes is analogous to the modulation data 92 delivered by the codegenerator 89 associated with each transmitter 17. In a manner which willbe explained, the digital code presented at port RB2 of microprocessor404 is imparted to the RF signal 18 emitted by wire loop 22 to permitreceiver/stimulator unit 14 to distinguish wire loop boundary 23 from aboundary 20 established by a transmitter 17.

As the flowchart of FIG. 12 illustrates, microprocessor 404 reads inputports RA0, RA1, RA2 and RA3 immediately following power-up to determinethe configuration of switch unit 408. Opening switches 407 and 414 whileclosing switches 412 and 413 cause microprocessor 404 to repetitivelydeliver the hexidecimal code "D9D9" at port RB2. Each bit of the codeappears at port RB2 for about 1.953 mSec before being replaced by thenext succeeding bit of the code. Thus, the entire sixteen bit code ispresented over an interval of about thirty-one and one-quartermilliseconds (31.25 mSec). Once the entire code has been presented, portRB2 assumes a low logic level for an inactive interval of aboutthirty-one and one-quarter milliseconds (31.25 mSec) before repeatingthe code. This cycle repeats as long as wire unit 21 remains energizedor until the configuration of switch unit 408 is changed. Alternatively,as FIG. 12 further shows, in the event switches 407, 412, 413 and 414all in an open position, microprocessor 404 repetitively generates thehexidecimal code "F590" at output port RB2 once every one hundred twentyfive milliseconds (125 mSec). Since the code itself is presented over aninterval of thirty-one and one-quarter milliseconds (31.25 mSec), aninactive interval of ninety-three and three-quarters milliseconds (93.75mSec) elapses before this code begins to repeat. Under such conditions,the output of port RB2 of microprocessor 404 is virtually identical tothe modulation data signal 92 associated with one of transmitters 17 andthe RF signal 18 emitted by wire loop 22 cannot be distinguished byreceiver/stimulator unit 14 from a similarly encoded RF signal 18emanating from a transmitter 17. This capability is useful in situationswhere it is not desired to distinguish wire loop boundary 23 from anelectronic boundary 20.

As a convenient means of verifying the operability of wire loop unit 21,a special testing mode can be established by closing switch 414 ofswitch unit 408. Closing switch 414 causes the voltage at input port RA3to assume a "high" or logic level "1" state. As FIG. 12 shows,microprocessor 404 reads port RA3 and determines whether or not it is ina high logic level state. If not, microprocessor 404 responds bydetermining further action based on the state of port RA0. If port RA3is at a logic "1" state, however, microprocessor 404 responds by writingthe hex code "D9D9" to output port RB2 three times only. Provided wireloop unit 21 and receiver/stimulator unit 14 are working properly, thisresults in delivery of a readily recognizable brief "burst" of audiostimulation from the audio transducer 315 of receiver/stimulator unit14.

With reference once again to FIG. 11A and FIG. 11B, it can be seen thatoutput Q2 at pin 7 of ripple counter 403 is connected by way of aresistor 421 to a tank circuit 422 which is coupled to ground by way ofa capacitor 424. Tank circuit 422 is tuned to resonate at the RF carrierfrequency of 8.192 KHz and consists of an inductor 426 connected inparallel with a capacitor 427. Tank circuit 422 resonates in response tothe 8.192 KHz delivered from output Z2 at pin 7 of ripple counter 403 togenerate a sinusoidal 8.192 KHz RF carrier signal 430.

Wire loop unit 21 modulates RF carrier signal 430 according to thedigital code generated at output port RB2 at pin 8 of microprocessor 404using binary phase shift keying (BPSK). To do so, wire loop unit 21 isprovided with an inverter U42A, and a pair of amplifiers U44A and U44Bwhose outputs are connected respectively to the line side (Y) of a pairof analog switches 433 and 434, respectively. Tank circuit 422 isconnected as shown to the noninverting input of amplifier U44B which isconfigured as a unity gain buffer by virtue of a connection between itsoutput and its inverting input. The output of amplifier U44B thusdelivers a sinusoidal RF carrier signal at zero phase shift (φ=0°) tothe line side (Y) of analog switch 433. In a complementary fashion, asinusoidal RF carrier signal at a relative positive phase shift ofninety degrees (φ=+90°) is delivered to the load side (Y) of analogswitch 434. This is achieved by connecting tank circuit 422 by way of acoupling capacitor 436 and a resistor 437 to the noninverting input ofamplifier U44A. Amplifier U44A has its inverting input connected to thecenter of a voltage divider formed of a pair of resistors 439 and 440connected in series with one another between supply rail 388 and circuitground to establish an offset voltage for amplifier U44A. Amplifier U44Ais configured as a unity gain, inverting amplifier by virtue of aresistor 441 connected between its output and its non-inverting input asshown. The output of amplifier U44A thus delivers to the line side (Y)of analog switch 434 a sinusoidal RF carrier frequency signal bearing apositive ninety degree phase shift to the corresponding signal presentat the line side (Y) of analog switch 433. Analog switches 433 and 434operate as normally open relays which close in response to applicationof a logic level signal at their respective control inputs (E) which arelocated at pins 5 and 13 of switches 433 and 434, respectively.

In order to generate a BPSK signal modulated with the code generated atoutput port RB2 at pin 8 of microprocessor 404, port RB2 is connecteddirectly to the control input (E) at pin 13 of analog switch 434 and isconnected by way of an inverter, U42A to the control input (E) at pin 5of analog switch 433. Inverter U42A serves to ensure that analogswitches 433 and 434 are never both open or both closed at the sametime. When the code signal appearing at output port RB2 at pin 8 ofmicroprocessor 404 is at a low logic level (i.e., data bit=0), analogswitch 434 opens and analog switch 434 closes. This causes the RFcarrier signal at zero degrees phase shift (φ=0°) to be applied to theload side (Z) of analog switch 433. Conversely, when the code signalappearing at output port RB2 at pin 8 of microprocessor 404 is at a highlogic level (i.e., data bit=1) analog switch 433 opens and analog switch434 closes. This causes the RF carrier signal at a positive ninetydegree phase shift (φ=+90°) to be applied to the load size (Z) of analogswitch 434.

To facilitate adjustment of the effective range of the RF signal 18emitted by wire loop 22 and thus, the effective width of wire loopboundary 23, wire loop unit 21 includes a potentiometer 444 which isconnected to a limiting resistor 445 as shown. Resistor 445 is connectedat a summing junction in series to the load side (Z) of analog switch434 by way of a coupling capacitor 447 and a series resistor 448.Resistor 445 is similarly connected in series with the load side (Z) ofanalog switch 433 by way of a coupling capacitor 449 and a seriesresistor 450. The common connection point of resistors 448 and 450defines a summing junction 451 at which the outputs of analog switches433 and 434 are effectively summed to define an 8.192 KHz sinusoidalBPSK signal whose instantaneous phase is correlated to the digital coderepetitively presented at output port RB2 of microprocessor 404. Byadjusting potentiometer 444, the amplitude of the BPSK signal applied tothe wiper of potentiometer 444 from summing junction 451 by way ofresistor 445 can be adjusted.

The wiper of potentiometer 444 is connected to the load side (Y) of ananalog switch 453 whose load side (Z) defines a node 455. In normaloperation, analog switch 453 remains closed so long as wire loop 22remains continuous thereby delivering the aforementioned BPSK signal tonode 455. Node 455 is in turn connected to the non-inverting input of anamplifier U47 by way of a coupling capacitor 459. The output ofamplifier U47 is connected in series with wire loop 22 by way of acoupling capacitor 461 and a current limiting resistor 462. Resistor 462is connected to the binding post 360 connected to one end of wire loop22. The opposite end of wire loop 22 is connected to circuit ground byway of binding post 361. A metal oxide varistor 464 is connected betweenbinding post 360 and circuit ground to protect wire loop unit 21 fromany high voltage transients which might be picked up by wire loop 22 inthe event of a lightning strike. Amplifier U47 serves to amplify thesinusoidal BPSK signal whose instantaneous phase varies in accordancewith the code data generated by microprocessor 404 at output port RB2.This amplified BPSK signal is then applied to wire loop 22 which servesas a transmitting antenna which broadcasts this digitally encoded BPSKsignal as an RF signal 18. While the RF signal 18 emitted by wire loop22 has a continuous sinusoidal component, the encoded portion thereofcontinuously repeats following brief delay intervals of about thirty-oneand one-quarter milliseconds (31.25 mSec). When potentiometer 444 is setsuch that RF signal 18 has a range establishing a wire loop boundary 23extending radially from wire loop a distance of about six feet, an RMScurrent of only about thirteen milliamperes flows through wire loop 22.Adjusting potentiometer 444 to increase that distance to about twentyfeet requires an RMS current of only about forty-four milliamperes. Thiscontrasts very favorably to the AM wire loop boundary systems of theprior art in which currents on the order of about eight hundredmilliamperes were required to be circulated through a wire loop toestablish an AM boundary field extending radially six to eight feet fromthe wire loop. Due to the dramatic reduction in operating currentrequired using the invention, back-up battery 372 allows wire loop unit21 to operate and wire loop boundary 23 to be maintained fully effectivefor sustained periods of time in the event of an A.C. power failure.

The continuity of wire loop 22 is monitored by a break detector circuit466 which is connected to wire loop 22 by way of a diode 468 whosecathode is connected to binding post 360 and whose anode is connected tothe center of a voltage divider consisting of a pair of resistors 469and 470. Resistor 469 serves as a pull-up resistor and is connected topositive supply rail 388 as shown. As long as wire loop 22 remainscontinuous, the anode of diode 468 remains at a voltage only slightlyabove circuit ground. In the event wire loop 22 breaks however, thevoltage at the anode of diode 468 rises to charge a capacitor 471 whichis connected in series between resistor 470 and circuit ground.

The voltage across capacitor 471 is monitored by an operationalamplifier U44C configured as a comparator. The non-inverting input ofamplifier U44C is connected to the positive side of capacitor 471 by wayof a resistor 473 and is connected to the output of amplifier U44Cthrough a feedback resistor 475. The inverting input of amplifier U44Cis connected to the center of a voltage divider consisting of a pair ofresistors 477 and 478 connected in series with one another betweensupply rail 388 and circuit ground as shown. Resistors 477 and 478 serveto maintain the inverting input of amplifier U44C at a predeterminedthreshold voltage. Amplifier U44C continuously compares that thresholdvoltage to the voltage at its non-inverting input to define a breakindicating signal at a "BREAK" terminal 480 at the output of amplifierU44C. In normal operation, wire loop 22 remains unbroken and the voltageacross capacitor 471 remains sufficiently below the constant thresholdvoltage at the inverting input of amplifier U44C to cause amplifier U44Cto hold the voltage at BREAK terminal 480 at a low logic level voltage.In the event the electrical continuity of wire loop 22 is brokenhowever, the voltage at the non-inverting input of amplifier risessufficiently to cause the output amplifier U44C at BREAK terminal 480 toassume a high logic level voltage.

Wire loop unit 21 includes an audio transducer 483 and a light emittingdiode (LED) 484 which provide audible and visual indications, of a breakin wire loop 22. For that purpose, BREAK terminal 480 is connected byway of a resistor 485 to the inverting input of an amplifier U44D. Thenon-inverting input of amplifier U44D is connected to a voltage dividernetwork consisting of a pair of resistors 486 and 487 and a capacitor488 connected as shown between supply rail 388 and circuit ground toestablish a threshold voltage. The inverting input of amplifier U44D isalso connected to the 8 Hz output Q12 at pin 12 of ripple counter 403 byway of a resistor 490. The output of amplifier U44D is connected to theinputs of a pair of hysteresis inverters U42B and U42C whose outputs areparalleled with one another and connected to the input of audiotransducer 483. The outputs of inverters U42B and U42C are alsoconnected to the inputs of a second pair of inverters U42F and U42Dwhose outputs are likewise paralleled and connected by way of a currentlimiting resistor 493 and LED 484 to circuit ground.

Provided loop wire 22 remains continuous, the voltage at the invertinginput of amplifier U44D always remains below the threshold voltageapplied to its non-inverting input. However, in the event of a break inwire loop 22, the voltage at BREAK terminal rises. The voltage at thenon-inverting terminal of amplifier U44D is then biased sufficiently tocause the output of amplifier U44D to toggle at an 8 Hz rate. Drivingamplifiers U42B, U42C and U42F, U42D cause audio transducer 483 to soundand LED 484 to flash at a corresponding rate to audibly and visiblysignal the breakage of wire loop 22.

Since it is not an AM signal, the RF signal 18 normally emitted by wireloop 22 cannot be detected by a conventional AM radio to locate the siteof a break in wire loop 22. To permit a user to locate a break in wireloop 22 using an AM radio, wire loop unit 21 includes facilities forselectively applying an AM signal to wire loop in the event such a breakis detected by break detector 466. For this purpose, wire unit 21includes a NAND gate U43C having a first input connected to BREAKterminal 480. NAND gate U43C includes a second input connected to theoutput of NAND gate U43A to receive the 32.768 KHz signal describedabove. The output of NAND gate U43C is thus normally at a constant lowlogic level state but oscillates at the rate of 32.768 KHz in the eventBREAK terminal 480 assumes a high logic level due to a break in wireloop 22. The output of NAND gate U43C is connected to a first input of aNAND gate U43B. NAND gate U43B also includes a second input which isconnected to output Q11 at pin 15 of ripple counter 403 to receive the16 Hz signal described above. Consequently, the output of NAND gate U43Bwill normally remain in a constant low logic level state but will takethe form of bursts of 32.768 oscillations repeating at a 16 Hz rate inthe event BREAK terminal 480 assumes a high logic level indicating abreak in wire loop 22. It will immediately be recognized that thepulsating 32.768 KHz signal generated by NAND gate U43B in the event ofa break in wire loop 22 constitutes a 32.768 KHz AM signal at onehundred percent (100%) modulation.

The output of NAND gate U43B is connected by way of a coupling capacitor495 to the center of a voltage divider consisting of a pair of resistors496 and 497 connected in series with one another between positive supplyrail 388 and circuit ground. Resistors 496 and 497 serve to apply a 2.5volt D.C. offset to the AM signal delivered from the output of NAND gateU43B. The junction between resistors 496 and 497 is connected to theline side (Y) of an analog switch 499. Analog switch 499 includes acontrol terminal (E) which is connected directly to BREAK terminal 480.The load size (Z) of analog switch 499 is connected to node 455. Thecontrol terminal (E) of analog switch 453 is connected to BREAK terminal480 by way of an inverter U42E. Since the voltage at BREAK terminal 480remains in a low logic level state as long as wire loop 22 remainscontinuous, analog switch 499 normally remains open while inverter U42Eapples a high logic signal to the control input (E) at pin 8 of analogswitch 453 thereby maintaining analog switch 453 closed as describedearlier. Under such normal operating conditions, analog switch 453serves to apply the BPSK signal received by the wiper of potentiometer444 by way of resistor 445 from node 451 to amplifier U47 which servesto deliver an amplified BPSK signal to wire loop 22 which emits acorrespondingly encoded RF signal 18.

In the event of a break in wire loop 22, however, BREAK terminal assumesa high logic level which causes analog switch 453 to open therebyterminating the application of the amplified BPSK signal to wire loop22. At substantially the same time analog switch 453 opens, analogswitch 499 closes thus delivering the AM signal described above to node455. This AM signal is then amplified by amplifier U47 and applied towire loop 22 by way of coupling capacitor 461 and resistor 462. Beingalerted to the breakage of wire loop 22 by the signals provided by audiotransducer 483 and LED 484, a user may then use a conventional AM radioto accurately locate the site of the break in wire loop 22. This can bedone by carrying an AM radio along the path of wire loop 22 andidentifying the site at which distinctive 16 Hz pulses are heard at amaximum audio volume.

A parts listing for the circuitry of the wire loop unit 21 of FIG. 11Aand FIG. 11B is set forth below in Table 3.

                  TABLE 3                                                         ______________________________________                                        Ref-            Quan-             Manufacturer's                              erence                                                                              Description                                                                             tity    Manufacturer                                                                            Part Number                                 ______________________________________                                        464   Metal Oxide                                                                             1       Panasonic ERZ-VO5D39                                        Varistor          Industrial Co.                                                                (Div. of                                                                      Matsushita                                                                    Electric Corp.                                                                of America)                                                                   (Secaucus NY)                                         468   Axial diode                                                                             1       Diodes Inc.                                                                             1N4148                                                              (Westlake Village                                                             CA)                                                   462   50 Ohm    1       ROHM Co., 50E-ND                                            leaded, 5%,       LTD.                                                        resistor          (Antioch TN)                                          360,  Two       1       Radio Shack                                                                             2741-621                                    361   terminal                                                                      mounting                                                                      post                                                                    444   10 Kohm   1       Clarostat Sensors                                                                       392JA103                                          potentio-         & Controls (El                                              meter             Paso TX)                                              484   Light     1       Interconnect                                                                            5100H5                                            emitting          Devices Inc.                                                diode             (Kansas City KS)                                      426   8.2 mH    1       J W Miller                                                                              5300-48                                           inductor          Magnetics                                                                     (Gardenia CA)                                         U43   Quad 2-input                                                                            1       National  CD4011BM                                          NAND gate         Semicondutor                                                                  Corp. (Santa                                                                  Clara CA)                                             403   12-stage  1       National  CD4040BCM                                         binary            Semiconductor                                               counter           Corp. (Santa                                                                  Clara CA)                                             408   Four position                                                                           1       CTS Corporation                                                                         CTS 206-4                                         DIP switch        (Elkhart IN)                                          368   Fullwave  1       DIODES INC                                                                              DB102-ND                                          diode bridge                                                            371,  470 uF    2       Panasonic ECE-A1EFS471                                461   leaded            Industrial Co.                                              capacitor         (Div. of                                                                      Matsushita                                                                    Electric Corp.                                                                of America)                                                                   (Secaucus NY)                                         386   10 uF     1       Panasonic ECS-F1CE106K                                      16VDC             Industrial Co.                                              capacitor         (Div. of                                                                      Matsushita                                                                    Electric Corp. of                                                             America)                                                                      (Secaucus NY)                                         483   3.7 KHz   1       Panasonic EFB-CB37C11                                       piezo             Industrial Co.                                              trancsducer       (Div.                                                                         of Matsushita                                                                 Electric Corp. of                                                             America)                                                                      (Secaucus NY)                                         U44A- Quad Low  1       National  LM324AM                                     U44D  power op          Semiconductor                                               amp               Corp. (Santa                                                                  Clara CA)                                             U47   amplifier 1       National  LM386                                                               Semiconductor                                                                 Corp. (Santa                                                                  Clara CA)                                             38    3-terminal, 5                                                                           1       National  LM7805CT                                          volt              Semiconductor                                               regulator,        Corp. (Santa                                                1 Amp             Clara CA)                                             372   Battery, 6                                                                              1                                                                   Volt                                                                          gel cell                                                                447,  1000 pF,  2       Surface   MCCE102J3NO                                 449   1206 Pkg.,        Mountable                                                   5%,               Electronic                                                  capacitor         Components, Inc.                                                              (Austin TX)                                           488   .01 uF, 1206                                                                            1       Surface   MCCE103J3NO                                       Pkg., 5%,         Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           459,  .1 uF, 1206                                                                             2       Surface   MCCE104J3NR                                 436   Pkg., 5%,         Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           400   20 pF, 1206                                                                             1       Surface   MCCE200J3NO                                       Pkg., 5%,         Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           424,  .22 uF, 1206                                                                            2       Surface   MCCE224J3NO                                 471   Pkg., 5%,         Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           495   39 pF, 1206                                                                             1       Surface   MCCE390J3NO                                       Pkg., 5%,         Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           399   5 pF, 1206                                                                              1       Surface   MCCE5R0C3NO                                       Pkg., .25%,       Mountable                                                   capacitor         Electronic                                                                    Components, Inc.                                                              (Austin TX)                                           497,  10 Kohm,  6       ROHM Co., MCR18-EZH-JW-                               473,  1206 Pkg.,        LTD.      103                                         440,  5%, resistor      (Antioch TN)                                          490,                                                                          485,                                                                          496                                                                           448,  100 Kohm, 2       ROHM Co., MCR18-EZH-JW-                               450   1206 Pkg.,        LTD.      104                                               5%, resistor      (Antioch TN)                                          475   1 Meg ohm,                                                                              1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      105                                               5%, resistor      (Antioch TN)                                          398   10 Meg    1       ROHM Co., MCR18-EZH-JW-                                     ohm,              LTD.      106                                               1206 Pkg.,        (Antioch TN)                                                5%, resistor                                                            470   1.3 Kohm, 1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      132                                               5%, resistor      (Antioch TN)                                          441,  15 Kohm,  8       ROHM Co., MCR18-EZH-JW-                               486,  1206 Pkg.,        LTD.      153                                         405,  5%, resistor      (Antioch TN)                                          406,                                                                          409,                                                                          410,                                                                          421,                                                                          411                                                                           477,  20 Kohm,  2       ROHM Co., MCR18-EZH-JW-                               445   1206 Pkg.,        LTD.      203                                               5%, resistor      (Antioch TN)                                          469   470 ohm,  1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      471                                               5%, resistor      (Antioch TN)                                          478   47K ohm,  1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      473                                               5%, resistor      (Antioch TN)                                          397   470 Kohm, 1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      474                                               5%, resistor      (Antioch TN)                                          493   510 ohm,  1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      511                                               5%, resistor      (Antioch TN)                                          487   5.1 Kohm, 1       ROHM Co., MCR18-EZH-JW-                                     1206 Pkg.,        LTD.      512                                               5%, resistor      (Antioch TN)                                          U42A- Hex Schmitt                                                                             1       National  MM74HC14M                                   U42F  Inverter          Semiconductor                                                                 Corp. (Santa                                                                  Clara CA)                                             433,  Quad      1       National  MM74HC4066N                                 434,  Bilateral         Semiconductor                                         453,  Switch            Corp. (Santa                                          499                     Clara CA)                                             427   .047 uF,  1       NIC Components                                                                          NSPC473J1683K                                     Temp. Stable      Corp. (Amityville                                           Cap               NY)                                                   404   8 bit     1       Microchip PIC16C54-LP\SO                          micro-            Technology, Inc.                                            processor         (Chandler AZ)                                         366   2.4 MM    1       CUI Stack, Inc.                                                                         PJ-002B                                           male jack         (Beaverton OR)                                        395   32.768 KHz                                                                              1       Raltron (Miami                                                                          R38-32.768 KHz                                    crystal           FL)                                                   369   0.5 Amp   1       Raychem Corp.                                                                           RXE110                                            Resettable        (Menlo Park CA)                                             fuse                                                                    ______________________________________                                    

Having described the structure and operation of wire loop unit 21 indetail, it is now appropriate to discuss the operation ofreceiver/stimulator 14 in further detail including the manner in whichthe microprocessor 281 of control and stimulation subsystem 130operates. Referring to FIG. 13, there is illustrated a Main Routinewhich is commenced by microprocessor 281, as indicated at block 500,immediately upon application of operating power to receiver/stimulatorunit 14 by installing battery 138. As indicated at block 502,microprocessor 281 determines the status of switch 310 by reading thelogic level status of ports RB6 and RB7 as described earlier andselecting an appropriate stimulation routine to be executed whenmicroprocessor 281 determines that one or more programmed preconditionsassociated with the reception of an RF signal 18 have been satisfied.Such preconditions include, but are not necessarily limited to, anindication that receiver/stimulator unit 14 has received an RF signal 18identified by a 16 bit (four digit hexidecimal) code which thatparticular receiver/stimulator unit 14 has been programmed to respond toby administering an aversive stimulus to animal 12. According to thestatus of ports RB6 and RB7 , microprocessor 281 selects a stimulationroutine for delivering either an audible tone only, an audible tonecombined with a high intensity ("High") electrical shock or an audibletone combined with a lower intensity ("Low") electrical shock. Within agiven stimulation routine, various stimulation parameters can beprogrammed to be selected based on which particular digital codereceiver/stimulator unit 14 detects at a given time. Such parametersinclude the duration of audio tone bursts and the lengths of timeintervals between successive bursts. Similarly, the repetition rate ofshock pulses 341, the number of shock pulses 341 and/or the length ofdelay intervals between successive series of shock pulses 341 can all bedetermined on a preprogrammed basis according to which particulardigital code is received by a particular receiver/stimulator unit 14 atany given time.

Once the appropriate stimulation routine is selected, microprocessor 241initializes a counter referred to by the mnemonic "MOTION" to an initialnumerical value as indicated at block 504. The purpose of doing so willbecome apparent upon explanation of the Motion Check subroutine of FIG.15. As block 506 indicates, microprocessor 281 then checks for a lowbattery condition and causes LED 308 to blink if battery 138 isdetermined to be low. This is accomplished simply by reading input portRB4 at pin 10 of microprocessor 281 and generating an intermittentoutput signal at output port RA2 in the event the status of port RB4indicates that low voltage detector 307 detects a voltage of less than2.35 volts at power supply rail 283. The stored operating program thencauses microprocessor 281 to successively call a Motion Check subroutineas indicated at block 508 and a Detect subroutine as indicated at block510. Program flow then loops back to block 506 as indicated torepetitively perform the battery check and the subroutines Motion Checkand Detect so long as operating power continues to be available toreceiver/stimulator unit 14. The Motion Check subroutine will now bedescribed with reference to FIG. 14.

The Motion Check subroutine serves to conserve battery 138 byselectively terminating the flow of operating power to portions ofreceiver/stimulator unit 14 during periods when animal 12 or 12' isdetermined not to be moving. As indicated at block 520, the Motion Checksubroutine starts immediately upon being called by the Main routine ofFIG. 13. As indicated at block 522, the status of motion switch 299 ischecked by reading the status of input port RB5 and storing this statusin a register designated by the variable name "Mx." As block 524indicates, microprocessor 281 then compares the present status ofregister Mx to the value, Mx-1, stored during the immediately precedingexecution of the Motion Check subroutine. If the two values differ, itis assumed that animal 12 is in motion. As blocks 526 and 528 show, thevalue of register MOTION is then checked and, provided this value doesnot exceed its predetermined maximum value of two-hundred fifty-five(255), the MOTION register is incremented. If the comparison of block524 indicates the status of motion switch 299 has not changed, animal 12is assumed not to be in motion. As blocks 530 and 532 indicate, thevalue of the MOTION register is then checked and decremented unless italready equals zero. After having been checked and, if necessary,incremented or decremented as a result of the execution of blocks 526and/or 528 or blocks 530 and/or 532, the status of the MOTION registeris again checked and compared with a predetermined numerical value suchas twenty as indicated at block 534. In the event the present valuestored in the MOTION register exceeds twenty, animal 12 is determined tobe sufficiently active to warrant monitoring. In that event, a logical"0" value is written to port RA2 as indicated at block 536, therebycausing FET 140 to either continue conducting or begin conducting theflow of current from battery 138 to those portions of receiver 133 whichconsume significant amounts of power. In the event FET 140 waspreviously non-conducting, a twenty millisecond delay loop is executedto allow receiver 133 to stabilize before attempting to detect anyencroachment of boundary 20 and/or 23. After block 536 is executed, theMotion Check subroutine ends as indicated at block 538 and program flowresumes with the Main Routine of FIG. 13. On the other hand, if thecomparison performed at block 534 indicates the present value of theMOTION register does not exceed the predetermined threshold value oftwenty, it is assumed that animal 12 has been sufficiently inactive asto warrant conserving power. In that event, as indicated at block 538,FET 140 is turned "off" by writing a logical "1" value to port RA2. Aone hundred fifty millisecond delay loop is then entered and the MotionCheck subroutine is again called in order to determine whether it is yetappropriate to turn FET 140 "on" and commence monitoring for anencroachment of a boundary 20 and/or wire loop boundary 20 due to anapparent resumption of movement on the part of animal 12. If and whenanimal 12 resumes movement, FET 140 will once again be turned on whenblock 536 is next executed. Upon ending the Motion Check subroutine atblock 538, the Main Routine of FIG. 13 resumes at block 510 by callingthe Detect Subroutine which will now be described with reference to FIG.15.

FIG. 15 summarizes the Detect subroutine which is executed bymicroprocessor 281 to determine whether certain preconditions associatedwith reception of an RF signal 18 have been satisfied and if so, toinitiate administration of an appropriate form of aversive stimulation.Two such preconditions are imposed to reduce the possibility that theanimal 12 will receive aversive stimulation even though it has notencroached upon any boundary 20 or 23. The first precondition which mustbe satisfied before microprocessor 281 will initiate any form ofaversive stimulation is reception of an RF signal 18 bearing a sixteenbit (four digit hexidecimal) code to which the particularreceiver/stimulator unit 14 has been programmed to respond. As notedpreviously, each receiver/stimulator unit may be programmed to respondto more than one such code according to preprogrammed Boolean criteria.

In order to determine whether and when such a first precondition issatisfied, the Detect subroutine starts as indicated at block 560 andproceeds immediately as indicated at block 562 to initialize twoseparate 16 bit (four digit hexidecimal) registers by loading each withdata such as "AAAA" which does not correspond to any valid codereceiver/stimulator unit 14 is to respond to but is otherwise arbitrary.These registers are referred to as "rolling" registers because they arecontinually updated with new data one bit at a time on a first-in,first-out (FIFO) basis. One of these rolling registers continuallyreflects the code data signal 275 presented to the input port RB2 at pin8 of microprocessor 281 while the other continually reflects thequadrature code data signal presented to the input port RB3 at pin 9 ofmicroprocessor 281. This is achieved as indicated at block 564 byreading the states of input ports RB2 and RB3 at least once during each1.953 millisecond interval depicted in FIG. 3, determining the value ofthe data during that interval and storing that data in a respective oneof each of the rolling registers. Each rolling register is updated inthis manner once every 1.953 milliseconds on a FIFO basis.

As block 566 indicates, microprocessor 281 next determines whether thedata presently appearing in either of the two rolling registerssatisfies a Boolean test and thus constitutes a "valid" code associatedwith any of the transmitters 17 whose RF signals 18 bear a code to whichthe particular receiver/stimulator unit 14 is programmed to respond. Ifonly one such valid code has been preprogrammed, the data in each of thetwo rolling registers is compared with that one code. If a plurality ofsuch codes have been programmed, microprocessor 281 compares each onewith the data in each of the two rolling registers and determineswhether a Boolean test defining one or some logical combination of thepreprogrammed codes is satisfied by the data in either of the tworolling registers. If such a Boolean test is satisfied, microprocessor281 determines that a valid code from a transmitter 17 has beenreceived. If no valid code associated with any of the transmitters 17 isdetected, it is assumed no applicable boundary 20 has been encroachedupon.

In such event, microprocessor 281 proceeds as indicated at block 568 todetermine whether the data in either of the two rolling registersconstitutes a valid code associated with a wire loop unit 21. Exceptthat the preprogrammed codes and/or the Boolean criteria applied may bedifferent, the execution of block 568 is otherwise the same as block 566as just described. If no valid code associated with a wire loop unit 21is detected, it is assumed no applicable wire loop boundary 23 has beenencroached upon. In such event, program flow continually loops back toblock 564 unless and until a valid code is detected upon a subsequentexecution of block 566 and/or block 568. If desired, the number of timesthe loop consisting of blocks 564, 566 and 563 can optionally be countedby a software counter. By comparing the present value of the counter toa predetermined upper limit and redirecting program flow to end theDetect subroutine when that upper limit is reached, it can be assuredthat the battery check of block 506 will be performed no less frequentlythan may be appropriate to detect a low battery condition before battery138 becomes unable to operate receiver/stimulator unit 14 reliably.

In the event microprocessor 281 determines as a result of the executionof block 566 and/or block 568 that a valid code has been received from atransmitter 17 and/or wire loop 22, a first precondition to theadministration of an aversive stimulus is deemed satisfied. However,microprocessor 281 preferably does not immediately initiateadministration of any stimulus. Rather, microprocessor 281 proceeds todetermine whether a second necessary precondition is also satisfied.According to this aspect of the invention, no stimulus will beadministered unless a subsequent transmission bearing the same code as aprevious transmission is received during a limited window of time whosewidth and spacing in time with respect to a prior transmission areconsistent with the duration and repetition rate known to be associatedwith transmissions emanating from a particular source. As the timingdiagram of FIG. 3 reflects, the hexidecimal code identifying atransmitter 17 repeats regularly beginning 93.75 mSec after itsimmediately preceding transmission has been completed. Likewise, asexplained above, the code associated with a wire loop unit 21 repeatsregularly beginning 31.25 mSec after its immediately precedingtransmission has been completed. By requiring a similarly encodedtransmission to be received during a limited window of time consistentwith one or the other of these known sets of timing parameters, thelikelihood of stimulating an animal 12 when no boundary 20 or 23 hasactually been encroached upon is greatly diminished.

In the preferred embodiment, this is achieved as shown in FIG. 15 bypromptly re-initializing both of the rolling registers as indicated atblock 570 as soon as a valid code associated with a transmitter 17 isdetected. A 93.75 millisecond delay is then executed as indicated atblock 572. As soon as the delay of block 572 is completed, input portsRB2 and RB3 of microprocessor 281 are read and the data so acquired isstored in each respective rolling register in the manner explained aboveas indicated at block 574. Once sixteen bits of data have been read andstored in each of the rolling registers in this manner, microprocessor281 compares the data in each of the two rolling registers with thevalid code data detected at block 566. If the code matches the data ineither of the two rolling registers, a second predetermined condition isdeemed satisfied whereupon both rolling registers are promptlyre-initialized as indicated at block 576 prior to initiatingstimulation. If the code does not match the data in either of the tworolling registers, the second predetermined condition is not satisfiedand no stimulation is initiated. Instead, the Detect subroutine ends asindicated at block 592 and program flow is redirected to block 506 ofthe Main routine of FIG. 13.

As block 578 indicates, microprocessor 281 next selects a stimulationroutine corresponding to the particular code determined to have beenreceived at blocks 566 and 574. In so doing, microprocessor 281 selectsat least one stimulation parameter in accordance with the particulardigital code received. As FIG. 10 reflects, the stimulation parametersso selected may include the duration of stimulation, the repetition rateof stimulation, the number of times stimulation is applied in responseto a single encroachment and/or the duration of intervals separatingsuccessive stimuli. Microprocessor 281 then executes the selectedstimulation routine by driving port RA1 or port RA1 and port RA0 togenerate the form and intensity of stimulation selected according to thestatus of switch 310 as determined at block 502 of the Main Routine ofFIG. 13. As block 578 indicates, stimulation is administered for aninterval of 93.75 milliseconds after which program flow is promptlyredirected to block 574 is indicated. Stimulation continues to beapplied in the manner determined by the selected stimulation routineuntil a valid code is no longer detected at block 574. In that event,the Detect subroutine ends as indicated at block 580.

Microprocessor 281 operates in a similar manner to determine whether asecond predetermined condition associated with reception of an RF signal18 from wire loop 22 is satisfied. As block 582 of FIG. 15 reflects,both rolling registers are promptly re-initialized as soon as a validcode associated with wire loop unit 21 is detected at block 568.Consistent with the 31.25 millisecond time interval between the end ofone transmission of the code associated with wire loop unit 21 and thebeginning of the next, a 31.25 millisecond delay is executed asindicated at block 584. As soon as that delay is completed, input portsRB2 and RB3 of microprocessor 281 are read and the data so acquired isstored in each respective rolling register as indicated at block 586.Once sixteen bits of data have been read and stored in each of therolling registers in this manner, microprocessor 281 compares the datain each of the two rolling registers with the valid wire loop codedetected at block 568. If that code matches the data present in eitherof the two rolling registers, the second predetermined condition isdeemed satisfied. In that event, both rolling registers arere-initialized as indicated at block 588. If there is no match, however,the second predetermined condition is not satisfied. Rather thaninitiating stimulation, the Detect subroutine ends as block 592indicates and program flow is redirected to block 506 of the Mainroutine of FIG. 13.

As block 590 reflects, microprocessor 281 then selects a stimulationroutine corresponding to the particular code determined to have beenreceived at blocks 568 and 586 and thereby selects one or morestimulation parameters such as those identified above. Microprocessor281 then executes the selected stimulation routine by driving port RA1or port RA1 and port RA0 to generate the form and intensity ofstimulation selected according to the status of selector switch 310 asdetermined at block 502 of the Main Routine of FIG. 13. As block 590indicates, stimulation is administered for an interval of 31.25milliseconds after which program flow is promptly redirected to block586 as indicated. Stimulation continues to be applied in the mannerdetermined by the selected stimulation routine until a valid code is nolonger detected at block 586. In that event, the Detect subroutine endsas indicated at block 592. Preferred stimulation routines areillustrated in FIG. 10. As noted previously, microprocessor 281 ispreferably programmed to include a software timer, loop counter or otherfacility to limit the maximum number of times and/or the maximumduration of stimulation delivered as the result of any single boundaryencroachment episode or series of episodes occurring closely in time toone another.

While the foregoing constitute preferred embodiments of the presentinvention, it is to be understood that the invention is not limitedthereto and that in light of the present disclosure, various alternativeembodiments will be apparent to persons skilled in the art. Accordingly,it is to be recognized that changes can be made without departing fromthe scope of the invention as particularly pointed out and distinctlyclaimed in the appended claims which shall be construed to encompass alllegal equivalents thereof.

What is claimed is:
 1. A radio frequency (RF) signaling apparatus,comprising:(a) a resonant circuit resonant substantially at an RFcarrier frequency, said resonant circuit including an antenna; and (b) adrive circuit coupled to said resonant circuit, said drive circuitdelivering to said resonant circuit, at substantially said RF carrierfrequency, brief pulses of electrical energy separated by intervals oftime in order to cause said resonant circuit to resonate, therebyexciting said antenna to transmit an RF signal at substantially said RFcarrier frequency.
 2. The apparatus of claim 1 wherein said drivecircuit comprises a circuit which delivers said pulses of electricalenergy at a duty cycle of less than about one percent (1%).
 3. Theapparatus of claim 1 wherein said drive circuit comprises a circuitwhich delivers said pulses of electrical energy at a duty cycle of lessthan about five percent (5%).
 4. The apparatus of claim 1 wherein saiddrive circuit comprises a circuit which delivers said pulses ofelectrical energy at a duty cycle of less than about fifteen percent(15%).
 5. The apparatus of claim 1 wherein said antenna is electricallysmall and said RF carrier frequency is a frequency of about tenkilohertz or less.
 6. The apparatus of claim 1 wherein said RF signalcomprises a signal having a magnetic field whose magnitude decreasessubstantially in proportion to the cube of increasing distance from saidantenna.
 7. The apparatus of claim 1 wherein said apparatus furthercomprises circuitry for controlling the phase of said pulses todigitally encode said RF signal.
 8. The apparatus of claim 1 whereinsaid RF signal comprises a signal having an amplitude bounded by anenvelope having a substantially constant magnitude.
 9. A radio frequency(RF) signaling apparatus, comprising:(a) a resonant circuit resonantsubstantially at an RF carrier frequency, said resonant circuitincluding an antenna; (b) a drive circuit coupleable to said resonantcircuit, said drive circuit delivering to said resonant circuit, atsubstantially said RF carrier frequency, pulses of electrical energyseparated by intervals of time in order to cause said resonant circuitto resonate, thereby exciting said antenna to transmit an RF signal atsubstantially said RF carrier frequency; and (c) an isolation circuitoperably interposed between said resonant circuit and said drivecircuit, said isolation circuit operating to effectively electricallyisolate said resonant circuit from said drive circuit during at least aportion of said intervals.
 10. The apparatus of claim 9 wherein saidportion of said intervals comprises substantially the entirety of saidintervals.
 11. The apparatus of claim 9 wherein said pulses are of aduration which is brief compared to said intervals and said RF carrierfrequency is a frequency of about ten kilohertz or less.
 12. Theapparatus of claim 9 wherein said RF signal comprises a signal having amagnetic field whose magnitude decreases substantially in proportion tothe cube of increasing distance from said antenna.
 13. The apparatus ofclaim 9 further comprising circuitry for controlling the phase of saidpulses to digitally encode said RF signal.
 14. The apparatus of claim 9wherein said RF signal comprises a signal having an amplitude bounded byan envelope having substantially constant magnitude.
 15. A radiofrequency (RF) signaling method, comprising the steps of:(a) providingan antenna resonant substantially at an RF carrier frequency; and (b)repetitively delivering to said antenna, at substantially said RFcarrier frequency, pulses of electrical energy separated by intervals oftime to cause said antenna to resonantly transmit an RF signal atsubstantially said RF carrier frequency.
 16. The method of claim 15wherein said pulses and said intervals represent a duty cycle of lessthan about one percent (1%).
 17. The method of claim 15 wherein saidpulses and said intervals represent a duty cycle of less than about fivepercent (5%).
 18. The method of claim 15 wherein said pulses and saidintervals represent a duty cycle of less than about fifteen percent(15%).
 19. The method of claim 15 further comprising the step ofencoding said RF signal by controlling the phase of said pulses.
 20. Themethod of claim 19 wherein said encoding step comprises the step ofshifting the phase of at least some of said pulses prior to deliveringsaid pulses to said antenna.
 21. The method of claim 15 wherein saidproviding step comprises the step of providing a said antenna which iselectrically small.
 22. The method of claim 15 further comprising thestep of encoding said RF signal by controlling the phase of said pulses.23. The method of claim 22 wherein said encoding step comprises the stepof shifting the phase of at least some of said pulses prior todelivering said pulses to said antenna.
 24. The method of claim 22wherein said providing step comprises the step of providing a saidantenna which is electrically small and wherein said RF carrierfrequency is a frequency of about ten kilohertz.
 25. A radio frequency(RF) signaling method, comprising the steps of:(a) providing an antennaresonant substantially at an RF carrier frequency; (b) providing a drivecircuit selectively electrically coupleable to said antenna; (c)repetitively delivering to said antenna, at substantially said RFcarrier frequency pulses, of electrical energy separated by intervals oftime to cause said antenna to resonantly transmit an RF signal atsubstantially said RF carrier frequency; and (d) effectivelyelectrically isolating said drive circuit from said antenna during atleast a portion of said intervals.
 26. The method of claim 25 whereinsaid portion of said intervals comprises substantially the entirety ofsaid intervals.
 27. The method of claim 25 wherein said pulses and saidintervals represent a duty cycle of less than about one percent (1%).28. The method of claim 25 wherein said pulses and said intervalsrepresent a duty cycle of less than about five percent (5%).
 29. Themethod of claim 25 wherein said pulses and said intervals represent aduty cycle of less than about fifteen percent (15%).
 30. The method ofclaim 25 further comprising the step of encoding said RF signal bycontrolling the phase of said pulses.
 31. The method of claim 30 whereinsaid controlling step comprises the step of shifting the phase of atleast some of said pulses prior to delivering said pulses to saidantenna.
 32. The method of claim 25 wherein said providing stepcomprises the step of providing a said antenna which is electricallysmall and wherein said RF carrier frequency is a frequency of about tenkilohertz or less.
 33. An apparatus for controlling the whereabouts ofan animal, said apparatus comprising:(a) at least one transmitteroperable to transmit a radio frequency (RF) signal defining a boundaryin the vicinity of said transmitter, said transmitter including:(i) aresonant circuit resonant substantially at an RF carrier frequency, saidresonant circuit including an antenna; and (ii) a drive circuit coupledto said resonant circuit, said drive circuit delivering to said resonantcircuit, at substantially said RF frequency, a series of pulses ofelectrical energy effective to cause said resonant circuit to resonatethereby exciting said antenna to transmit said RF signal; (b) a receiverfor receiving said RF signal; and (c) a stimulator responsive to saidreceiver for determining based on said RF signal whether at least onepredetermined condition indicating encroachment of said boundary by theanimal has been satisfied and, subject to satisfaction of saidcondition, administering to the animal at least one stimulus fordeterring said encroachment in order to control the whereabouts of theanimal.
 34. The apparatus of claim 33 wherein said RF carrier frequencyis a frequency of about 10 kilohertz or less.
 35. The apparatus of claim33 wherein said antenna is electrically small and said RF carrierfrequency is a frequency of about ten kilohertz or less.
 36. Theapparatus of claim 33 wherein said transmitter is a transmitter operableto transmit a said RF signal having a magnetic field whose magnitudedecreases substantially in proportion to the cube of increasing distancefrom said transmitter.
 37. The apparatus of claim 33 wherein saidtransmitter further comprises circuitry for controlling the phase ofsaid pulses to digitally encode said RF signal.
 38. The apparatus ofclaim 33 wherein said transmitter is a transmitter operable to transmita said RF signal having an amplitude bounded by an envelope ofsubstantially constant magnitude.
 39. An apparatus for controlling thewhereabouts of an animal, said apparatus comprising:(a) at least onetransmitter operable to transmit a first radio frequency signal whichdefines a first boundary in the vicinity of each said transmitter; (b) awire loop operable to transmit a second radio frequency signal whichdefines a second boundary in the vicinity of said wire loop; (c) areceiver for receiving said first and second radio frequency signals;and (d) a stimulator operably coupled to said receiver for determining,based upon at least one of said first and second signals, whether atleast one predetermined condition indicating encroachment of at leastone of said first and second boundaries by the animal has been satisfiedand, subject to satisfaction of said condition, administering to theanimal at least one stimulus for deterring said encroachment in order tocontrol the whereabouts of the animal.
 40. The apparatus of claim 39wherein said second radio frequency signal is of a frequency of lessthan about ten kilohertz.
 41. The apparatus of claim 39 wherein saidsecond radio frequency signal is a signal whose amplitude is bounded byan envelope of substantially constant magnitude.
 42. The apparatus ofclaim 39 wherein said first radio frequency signal and said second radiofrequency signal each bear a digital code, the reception of whichconstitutes a said predetermined condition.
 43. The apparatus of claim39 wherein said first radio signal is a signal having a magnetic fieldwhose magnitude decreases substantially in proportion to the cube ofincreasing distance from said transmitter.
 44. An apparatus forcontrolling the whereabouts of an animal, said apparatus comprising:(a)at least one transmitter operable to transmit a radio frequency signaldefining a boundary in the vicinity of said transmitter, said radiofrequency signal having a magnetic field the magnitude of whichdecreases substantially in proportion to the cube of increasing distancefrom said transmitter; (b) a receiver for receiving said signal; and (c)a stimulator responsive to said receiver for determining based on saidsignal whether at least one predetermined condition indicatingencroachment of said boundary by the animal has been satisfied and,subject to satisfaction of said condition, administering to the animalat least one stimulus for deterring said encroachment in order tocontrol the whereabouts of the animal.
 45. The apparatus of claim 44wherein said transmitter comprises a transmitter operable to transmitsaid radio frequency signal at a frequency of about ten kilohertz orless.
 46. A method for controlling the whereabouts of an animal, saidmethod comprising the steps of:(a) providing a wire loop along a pathdefining a boundary for the animal; (b) coupling the loop to a source ofradio frequency electrical energy effective to cause the loop totransmit a radio frequency signal other than an amplitude modulated (AM)signal; (c) detecting a condition of said radio frequency signalindicating encroachment of said boundary by said animal and, in responseto said detecting of said condition, administering to the animal atleast one stimulus for deterring the animal from said encroachment; (d)monitoring for the occurrence of a break in said loop; and (e) inresponse to detecting a said break in said loop, selectively couplingthe loop to a source of amplitude modulated (AM) radio frequencyelectrical energy whereby locating the site of said break using aconventional AM receiver is facilitated.
 47. In a system for controllingthe whereabouts of an animal of the type in which an electronic boundaryfor the animal is established by transmitting a radio frequency signaland at least one aversive stimulus is delivered to the animal inresponse to detecting a condition of the radio frequency signalindicating that the boundary has been encroached upon by the animal, theimprovement comprising:interposing inactive intervals between at leastsome successive transmissions of said radio frequency signal.
 48. Thesystem of claim 47 wherein said inactive intervals are each within arange of from about thirty milliseconds to about five seconds.
 49. Anapparatus for controlling the whereabouts of an animal, said apparatuscomprising:(a) a transmitter for intermittently transmitting radiofrequency signals defining a boundary for the animal; (b) a receiver forreceiving said signals; and (c) a stimulator operably coupled to saidreceiver for administering at least one stimulus to the animal to deterthe animal from encroachment of said boundary in response todetermining, based on said radio frequency signals, whether at least onepredetermined condition indicating encroachment of said boundary hasbeen satisfied, at least one said predetermined condition beingreception of at least one of said radio frequency signals during apredetermined limited window of time occurring subsequent to receptionof a previously transmitted one of said radio frequency signals.
 50. Amethod for controlling the whereabouts of an animal, said methodcomprising the steps of:establishing a boundary for the animal byintermittently transmitting radio frequency signals in the vicinity ofthe boundary; receiving said signals; and selectively administering tothe animal at least one stimulus to deter the animal from encroachmentof said boundary in response to determining, based on said signals,whether at least one predetermined condition indicating encroachment ofsaid boundary has been satisfied, at least one said predeterminedcondition being reception of at least one of said signals during alimited window of time occurring subsequent to reception of a previouslytransmitted one of said signals.
 51. An apparatus for controlling thewhereabouts of an animal, said apparatus comprising:(a) a transmitterfor intermittently transmitting radio frequency signals bearing apredetermined code, said signals defining a boundary for the animal; (b)a receiver for receiving said signals; and (c) a stimulator operablycoupled to said receiver for administering at least one stimulus to theanimal to deter the animal from encroachment of said boundary inresponse to determining, based on said signals, whether at least onepredetermined condition indicating encroachment of said boundary hasbeen satisfied, at least one said predetermined condition beingreception of at least one of said signals during a predetermined limitedwindow of time occurring subsequent to reception of a previouslytransmitted one of said radio frequency signals, an additional saidpredetermined condition being reception of at least one of said signalsbearing said predetermined code.
 52. A method for controlling thewhereabouts of an animal, said method comprising the stepsof:establishing a boundary for the animal, said establishing stepincluding the step of intermittently transmitting radio frequencysignals in the vicinity of the boundary, at least some of said signalsbearing a predetermined code; receiving said signals; and selectivelyadministering to the animal at least one stimulus to deter the animalfrom encroachment of said boundary in response to determining, based onsaid signals, whether at least one predetermined condition indicatingencroachment of said boundary has been satisfied, at least one saidpredetermined condition being reception of at least one of said signalsduring a limited window of time occurring subsequent to reception of apreviously transmitted one of said signals, an additional saidpredetermined condition being reception of at least one of said signalsbearing said predetermined code.
 53. An apparatus for controlling thewhereabouts of animals, said apparatus comprising:a first deviceoperable to transmit a first radio frequency signal defining a firstboundary for the animal, said first radio frequency signal bearingpredetermined first information; a second device operable to transmit asecond radio frequency signal defining a second boundary for the animal,said second radio frequency signal bearing predetermined secondinformation; and a unit affixable to the animal operable to receive atleast one of said first and second radio frequency signals, and todetermine, based on at least one of said first information and saidsecond information, whether at least one predetermined conditionprecedent to applying a stimulus to the animal to deter the animal fromencroaching at least one of said first boundary and said second boundaryis satisfied.
 54. The apparatus of claim 53 wherein said first devicecomprises a wire loop whose path traverses the midst of at least aportion of said first boundary.
 55. The apparatus of claim 53 whereinsaid second device comprises a battery powered transmitter.
 56. Theapparatus of claim 53 wherein said first information comprises a firstdigital code and said second information comprises a second digital codedistinguishable from said first digital code.
 57. The apparatus of claim53 wherein said unit comprises a unit further operable to select atleast one predetermined stimulation parameter based on at least one ofsaid first and second information.